Light waves
Reflection of Light waves
Subject: Physics
Theme: Waves-Motion Without Material Transfer
Topic: Light waves
Sub Topic: Reflection of Light waves
Date: dd/mm/yyyy
Class: S.S.S 2
Duration: 40 Minutes
No of Learners: 40
Learning Objectives: By the end of the lesson learners should be able to:
1. Defined Light or light waves
Light is defined as an electromagnetic wave with a wavelength in the visible part of the electromagnetic spectrum (380 to 700 nanometres). That is, light is any electromagnetic wave that we can see with our eyes.Light wave is a visible source of energy which is radiated outward from a source. It is also a wave motion. It has a very short wavelength of 5Γ10-4 mm. Light travels at a speed of 3.0Γ108 m/s
2. List and explain Source of Light waves
There are various sources of light: natural and artificial, luminous and non-luminous.(i) Natural sources: Natural sources of light include the sun and the stars.
(ii) Artificial sources: Artificial sources of light are the candle, electric torch, the electric lamp, incandescent, arc light and fluorescent light.
(iii) Self-luminous or luminous object: Self-luminous or luminous sources of light are those that generate and emit light by themselves e. g. the sun, stars, fire flies and some deep sea fishes.
(iv) Non-luminous objects: Non-luminous objects are seen when they reflect or throw back light from a luminous objects. Examples of non-luminous objects are moon, paper, mirror, wall etc. When light falls on such surface, it may be absorbed, transmitted or reflected, sometimes a combination of the above processes may occur.
3. Explain Light and Matter:
Light interacts with matter in various ways. When light strikes an object, it can be reflected (bounce off), refracted (change speed and direction), absorbed (take it in), or transmitted (pass through), and emitted (give it off). The color of an object depends on which colors of light are absorbed and which are reflected.4. Explain Transmission of Light
Transmission of light occurs when it passes through a material without being absorbed.(i) Transparent materials: Transparent materials allow light to pass through with little or no scattering, making them see-through. Examples of transparent materials include glass, air and clear plastics. Transparent materials transmit light waves without distorting images.
Spectrum When a narrow beam of light passes through a glass prism, rainbow of different colour can be seen.
(ii) Translucent materials: Translucent materials transmit light waves but objects cannot be seen clearly. Example, frosted glass/window, frosted light bulbs, translucent/foggy glass. Etc.
(iii) Opaque materials: Opaque materials transmit no light but absorbs and reflects all light incident upon them. Shadows/Eclipses are formed due to an opaque materials.
A shadow consists of two parts:
The umbra, where no light reaches, and the penumbra, where some light is partially absorbed (blocked). The size and sharpness of a shadow depend on the size and distance of the light source.
Eclipses are natural phenomena that occur when one celestial body, such as the Moon or Earth, passes through the shadow of another celestial body.
Solar eclipses happen when the Moon passes between the Sun and Earth, blocking out (absorbing) the Sun's light.
Lunar eclipses occur when Earth comes between the Sun and the Moon, casting a shadow on the Moon.
5. Differentiate between Light rays and beams:
(i) A ray is the direction of the path in which light is travelling. It is represented by a straight-line with an arrowhead(ii) A beam is a collection of two or more rays of light. Beams can be parallel, convergent or divergent.
(a) A parallel beam is two or more rays travelling in the same direction but can never intersect each other.
(b) A beam of light is said to be convergent when they meet at a point
(c) The divergent beam occurs when a collection of light rays has the same source is spread out apart.
6. Explain RECTILINEAR PROPAGATION OF LIGHT
The phenomenon of light travelling in straight line is known as rectilinear propagation of light.(1) It can be demonstrated by placing a candle flame at the end of a straight pipe, light of the flame will be seen clearly at the other side of the pipe. If the pipe is then bent and the process repeated, nothing will be seen at the other end, this clearly shows that light travels in straight line.
(2) Another demonstration of rectilinear propagation of light is by placing three equally sized cardboards with a tiny hole in the center and arranged in a straight line in front of each other. A string is passed through the holes and pulled taut to ensure that the holes are aligned in a straight line. A light from a candle is placed at one end of the row of cards. When we look through the other end of the row, we will see the light from the candle flame through the holes. If either cardboard is slightly shifted out of the straight line position, the light is cut off.
(3) Two natural effects that result from the rectilinear propagation of light are the formation of eclipse and shadow, The principle of operation of the pin hole camera also depends on the fact that light travels in straight lines.
SHADOW A shadow is an area in which light rays from a source cannot reach. It is produced by the obstruction of light by an opaque object. There are two types of shadow: partial (penumbra) shadow and total (umbra) shadow. If the light source is large, the shadow formed consist of two parts, a completely dark area known as umbra and an outer grey area known as penumbra or partial shadow. In the umbra region, the light from the source is completely blocked by the opaque body. In the penumbra region, the light is partially blocked by the opaque object. The inner region of the shadow receives less than the outer parts. Thus the penumbra becomes brighter from the umbra and outwards.
ECLIPSE An eclipse is a result of a shadow cast by one heavenly body on another. The sun being a luminous body and it is in the middle while the earth and the moon revolves round the sun. If the moon is between the sun and the earth, the shadow of the moon will be cast on the earthβs surface. There are two types of the eclipse. Viz:
1. Eclipse of the sun (solar eclipse): here the moon comes between the sun and the earth in a straight line
2. Eclipse of the moon (lunar) eclipse: in this case, the earth comes in between the sun and the moon.
(4) Pin Hole Camera: Another fact that shows that light travels on a straight line is demostrated with the pin hole camera.
A pinhole camera, also known as a camera obscura, is a simple device that uses a small hole to project an inverted image of the outside world onto the opposite wall of a darkened room or surface. The pinhole allows only a limited amount of light to enter, and the inverted image is formed due to the rectilinear propagation of light.
It consists of a light proof box, one end of which has a small hole made with a pin or needle point. The opposite end has a screen made with tracing paper or ground glass. Light from an object in front of the pinhole passes through it and form an image on the screen. If the screen is replaced with a photographic paper or film, a picture of the object can be taken with the pinhole camera. When using the pinhole camera to take pictures of an object, long exposure is necessary to allow sufficient light to enter the box through the pin hole. The image formed on the screen of the pinhole camera will be seen more clearly if external light is excluded by covering head and camera with a dark cloth. The image formed on the screen of the pinhole camera is inverted.
7. Solve calculations on magnification of Image on pin hole cameral.
Linear magnification: Magnification is defined as the ratio of the size (or height) of the image to the size (or height) of the objectππππππ ππππππππππ‘πππ (m) = πππππ π ππ§π (hi) / ππππππ‘ π ππ§π (ho)
= πππππ πππ π‘ππππ from pinhole (v) / ππππππ‘ πππ π‘ππππ from pinhole (u)
= length of camera / distance of object from pinhole
= height of image / height of object
m = hi / ho = v/u
EXAMPLE 1.
The height of an object place at a distance of 60 cm from the hole of a pinhole camera is 15 cm. If the length of the cameral is 10 cm, calculate the magnification produced by the cameral and the height of the image.
SOLUTION
m = length of cameral / distance of object from pinhole
m = 10 / 60 m = 0.1666
Also m = height of image (h') / height of object (h)
0.1666 = h' / 15 cm
h' = 2.5 cm
EXAMPLE 2.
What is the width of the image of an object which subtends an angle of 80 at the pinhole of a camera 0.5 m long? How wide is the image of the sun in this camera. Take the mean distance of the Sun as 1.5 x 1011 m, and diameter of the sun as 1.4 x 109 m
SOLUTION
(i) Let width of image = h' (m)
tan 80 h/0.5 m
h = 0.5 x tan 80
h = o.5 x 0.1405 m
h = 7 cm
(ii) Let radius of image of the sun be s' (m)
tan ΞΈ = s'/ 0.5 = 0.7x109 / 1.5x1011
s' = (0.5 x 0.7x109)/1.5x1011
s' = 0.233x10-2 m
s' = 0.23 cm
:. diameter of the image of the sun = 2 x 0.23
Image of the sun = 0.46 cm
EXAMPLE 3
The sun is just covered by a disc of 2cm diameter placed about 2m from the eye. If the length of the placed of the suns image formed by a pin-hole camera is 0.5cm, calculate the distance from the pin-hole to the screen.
SOLUTION
Hβ = 2cm
Dβ = 2m = 20cm
Hβ = 0.5cm
Dβ = ?
m = Hβ / Hβ = Dβ / Dβ
0.5 / 2 = Dβ / 200
Dβ = 0.5 x 200 / 2
Dβ = 50cm
Dβ = 0.5m
EXAMPLE 4
A small hole is made up of a window shutter of a room, 10.5m wide and the image of a tree outside the room cast on the opposite. If the image is 4.5m high and the tree is 30.5cm from the window, what is the height of the tree?
SOLUTION
Image height (Hβ) = 4.5m
Object height (Hβ) = ?
Image distance (V) = 10.5m
Object distance (U) = 30.5cm
m = Hβ / Hβ = V / U
4.5 / Hβ = 10.5 / 30.5
Hβ = 4.5 x 30.5 / 10.5
Hβ = 13.07m
8. Explain Reflection of Light waves on plane mirror/surface.
Reflection is the bouncing back of light waves when it strikes a surface.(a) Reflection of plane mirrors
There are two types of reflection:1. Regular Reflection
2. Diffuse Reflection or Irregular Reflection
(1) In regular reflection, parallel rays of light incident on a smooth or polished surface are reflected as parallel rays in one direction.
(2) In diffused or irregular reflection, parallel rays of light incident on a rough or irregular surface are reflected in various directions.
9. State the laws of reflection.
LAWS OF REFLECTIONThe first law of reflection states that the incident ray, the reflected ray and the normal at the point of incidence all lie on the same plane
The second law of reflection states that the angle of incidence (i) is equal to angle of reflection (r).
10. List the characteristics of image formed by plane mirror.
CHARACTERISTICS OF IMAGE FORMED BY PLANE MIRROR2. It is virtual.
3. It is laterally inverted.
4. It is upright.
5. It is far behind the mirror as the object is in front of the mirror.
There are two types of image:
1. Real image
2. Virtual image
(1) A real image is one that can be caught on a screen. Light rays actually pass through real image.
(2) A virtual image is one that cannot be caught on a screen. It is one through which rays do not actually pass but which is nevertheless visible to the eye.
LATERAL INVERSION: The effect on plane mirror on objects placed in front of it whereby the appearance of the image looks like a reversal of the object is known as lateral inversion.
IMAGES FORMED BY INCLINED MIRROR: When two mirrors are placed at an angle to each other, the number of images formed is given by:
π = (3600 / π) β 1
π = Number of images
Σ¨ = Angle of inclination
When Σ¨ = 1800, the two mirrors will act as a single mirror and therefore formed only one image. When Σ¨ = O, the two mirrors are parallel to each other and the image of object placed between them will be at infinity.
i.e
π = (3600 / 0) - 1
π = β
EFFECT OF MIRROR ROTATION ON REFLECTED RAY-MIRRO GALVANOMETER: If the direction of an incident ray on a mirror is kept constant and the mirror is rotated through twice that angle. This fact is utilized in mirror galvanometer (to measure very small electric current) and in the navigatorβs sextant.
Example
The reflection of a narrow beam of light incident normally on a plane mirror falls on a metre rule parallel to the mirror and at a distance of 1m. Calculate the angle of rotation of the mirror if the reflected beam is displaced 21.26cm along the metre-rule when the mirror rotated.
SOLUTION
Incident ray = ON
reflected ray before rotation = ON
reflected ray after rotation = NP
let the mirror be rotated through ΞΈ
reflected ray is rotated through 2ΞΈ
i.e ONP = 2ΞΈ
Tan 2Σ¨ = 21.26/1 m
Tan 2Σ¨ = 21.26/100 cm
Tan 2Σ¨ = 0.2126
2Σ¨ = tan-1 0.2126
2Σ¨ = 120
Σ¨ = 120/2
Σ¨ = 60
1. It is used in periscope
2. It is used in kaleidoscope
3. It is used in sextant
11. Explain Reflection of Light waves on curved mirrors.
Curved mirrors differ in size, shape and direction of their curvature.In respect of shape, we have spherical and parabolic mirrors.
There are two types of spherical mirrors: concave and convex mirrors.
1. Concave mirrors
The concave mirrors are hollowed-out toward the incident light like the inside surface of a spoon. It is also called a converging mirror.2. Convex mirrors
These mirrors bulge towards the incident light like he back of a spoon. Convergent mirrors are also referred to as divergent mirrors.3. Parabolic mirrors
These is a concave mirror adopted to focus any beam of light originating at a single point, no matter how close or far away from the source of light is from the mirror. Parabolic mirrors are used in solar cookers to focus the sun's rays and in car headlight to create powerful beam of light from a single light globe.In car headlamps the light from the bulb (filament or LED) is collected by the mirror and reflcted to produce an approximately parallel beam of rays to illuminate a narrow field of view ahead of the vehicle, the mirror acts with a small angle of divergence.
List the USES OF CURVED MIRROR:
CONCAVE MIRROR
1. It is used in vehical headlights
2. It is used in searchlight
3. It is used in flashlights
4. It is used in makeup mirrors
5. It is used in shaving mirrors
6. It is used in microscope
7. It is used in telescope
8. It is used n solar cookers.
CONVEX MIRROR
1. It is used in sunglasses
2. It is used in rear-view mirrors in automobile
3. It is utilized in ATMs and other plases for security reasons
4. It is used as a reflectors for street lights
Terms used with spherical mirrors
1. The pole (p) β this is the midpoint of the spherical mirrors
2. The aperture β this is the width or diameter of the mirror.
3. The center of curvature (c) β this is the centre of the large sphere from which the spherical mirror is carved out.
4. The radius of curvature (R) β this is the distance between the center of curvature and the pole of the mirror.
5. The principal axis β this is the imaginary line passing through the pole (p) and the center of curvature (c)
6. The principal focus (f) β this is the point on the principal axis where the incident rays converges (for concave mirrors) or appear to diverge (for convex mirror)
7. Focal length (f) β this is the distance between the focus and the pole of the spherical mirror. It is always half of radius of curvature
π = π /2
π = 2π
Spherical aberration
This is the phenomenon whereby a spherical mirror of wide aperture cannot bring all parallel rays to the same focus. In other to avoid this, spherical mirrors of small aperture are usually used. This is also why parabolic mirrors are used in place of spherical mirrors in searchlights and car headlamps.
12. Construction of ray diagrams.
The following tips are used in constructing ray diagramsi. Light rays parallel to principal axis are reflected through the focus
ii. A light ray passing through the center of the curvature is reflected back along the same path
iii. A light ray passing through the focus is reflected parallel to the principal axis.
iv. Light rays striking the mirror at the pole is reflected such that the angle of incidence is equal to the angle of reflection.
NOTE: When rays are produced behind the mirror, they are indicated using dotted lines. This means that they are imaginary or virtual. Hence the focal point and focal length of a concave mirror are real while the focal point and focal length of a convex mirror are virtual. A real focal length is given a positive sign while a negative focal length is given a negative sign.
We can locate the image formed by a curved mirror graphically by represently the sperical mirror by a straight line instead of a curved one.
Example
By means of an accurate graphical construction, determnine the position, size and nature of the image of an object 5 mm tall, standing on the principal axis of a concave mirror of focal length 20 mm and 34 mm from the mirror.
Solution
Please Note. As Best Practice: All data given and measurements made should be recorded on the grahp sheet.
Characteristics of image formed by concave mirrors
a. Object before center of curvature:the image formed is:
β’ diminished
β’ between the center of curvature and the focus
β’ inverted
β’ real
b. Object at the center of curvature:
the image formed is:
β’ same size the object
β’ at the center of curvature
β’ inverted
β’ real
c. Object between the center of curvature and the focus:
the image formed is:
β’ Magnified
β’ Beyond the center of curvature
β’ Inverted
β’ real
d. Object at focus:
the image formed is:
β’ Formed at infinity
e. Object between focus and the pole:
the image formed is:
β’ Magnified
β’ Behind the mirror
β’ Virtual
β’ Erect
f. Object at infinity:
the image formed is:
β’ Diminished
β’ Formed at the focus
β’ Real
β’ Inverted
Characteristics of image formed by convex mirrors
The image formed by a convex mirror is always:β’ Virtual
β’ Erect and
β’ Diminished in size
β’ It is formed between the pole and the principal focus.
This is unlike the case of the concave mirror which can produce either real or virtual images that may be inverted or erect, magnified or diminished in size according to the position of the object.
| The table below provides a summary of how a concave and convex mirror forms images | |||
|---|---|---|---|
| S/N | POSITION OF THE OBJECT | IMAGE FORMATION BY CONCAVE MIRROR | IMAGE FORMATION BY CONVEX MIRROR |
| 1 | Object at infinity | Image formed is inverted, real, diminished and formed at F | Image formed is upright, virtual and diminished |
| 2 | Object beyond C | Image formed is real, inverted and diminised | Image formed is upright, virtual and diminished |
| 3 | Object at C | Image formed is real, inverted and same size as the object | Image formed is upright, virtual and diminished |
| 4 | Object between C and F | Image formed is real, inverted and magnified | Image formed is upright, virtual and diminished |
| 5 | Object at F | Image formed is real, inverted and at infinity | Image formed is upright, virtual and diminished |
| 6 | Object between F and P | Image formed is virtual, upright and magnified | Image formed is upright, virtual and diminished |
| Note that a concave mirror always forms real and inverted and images except when the object is placed between the focal point and the pole of the mirror when it forms a virtual and inverted image. On the other hand, a convex mirror always forms a virtual, erect and diminished image. A real image is that image formed by actual intersection of real rays while a virtual image is formed by imaginary rays. Furthermore, a real image can be formed on a screen while a virtual image cannot be formed on a screen. |
|||
11. state the Mirror formula & Sign convention and use them to solve problems
1. The Mirror formula: The focal length, f, object distance, u, and the image distance, v, can be related using the formula below:1/π’ + 1/v = 1/π -----(1)
(v + u)/uv = 1/f
uv/(v + u) = f
π = π’π£/(π’+π£) ----- (2)
From Equation 1
1/π’ = 1/π - 1/v
1/u = (v - f)/fv
π’ = ππ£/(π£ β π) -----(3)
Similarly
π£ = ππ’/(π’ β π) -----(4)
From Linear magnification that was defined as the ratio of the image size to the object size
ππππππ ππππππππππ‘πππ = πππππ hπππhπ‘ / ππππππ‘ hπππhπ‘ = πππππ πππ π‘ππππ / ππππππ‘ πππ π‘ππππ
π = hπ / hπ = π£ / π’
π£ = ππ’ -----(5)
π’ = π£/π -----(6)
2. Sign convention: This is used to know and calculate by properly assigning sign to all the parameters used in mirror.
(i) The new Cartesian: Here, all the distances measure to the left of the mirror from the pole are negative while distances measured to the right of the mirror from the pole are positive
(ii) Real is positive and virtual is negative: This is the most widely accepted and used in calculations for mirrors and lenses. In this case:
β’ All distances are measured from the pole of the mirror to either left or right
β’ The distance of real objects and real images are positive
β’ The distance of virtual objects and virtual images are negative
β’ The focal length of a concave mirror is positive while the focal length of a convex mirror is negative
Example 1.
An object which is 5.0cm high is placed 10.0cm in front of a convex mirror of focal length 15.0cm. Find the position, size and nature of the image produced.
Solution
Using βreal is positiveβ Given that f=-15cm, u=10cm
1/π’ + 1/v = 1/π
1/10 + 1/π£ = β1/15
1/π£ = β(1/15 + 1/10)
1/π£ = β(5/30)
π = β6.0ππ
For magnification
π = π£/π’
π = β6/10
π = β0.6
πΌππππ π ππ§π = ππππππππππ‘πππ Γ ππππππ‘ π ππ§π
πΌππππ π ππ§π = ππππππππππ‘πππ Γ ππππππ‘ hπππhπ‘
πΌππππ π ππ§π = 0.6 Γ 5.0
πΌππππ π ππ§π = 3.0ππ
Thus, the image is formed 6.0cm behind the mirror and the height 3.0cm. it is erect, virtual, diminished
Example 2.
A concave mirror of radius of curvature 20cm produces an inverted image 3 times the size of an object place on and perpendicular to the axis.
Calculate the position of the object and image.
Solution
m = 3, r = 20cm
f = r/2 = 20/2
f = 10cm
m = v/u
3 = v/u
v = 3u
Image Formula
1/f = 1/u + 1/v
1/10 = 1/u + 1/3u
1/10 = (3+1)/3u
1/10 = 4/3u
3u = 40
u = 40/3
u = 13.3cm
But v = 3u
v = 3x13.3
v = 39.9cm
v β 40cm
:. Position of the object is 13.3cm from the mirror, position of the image is 40cm from the mirror
Example 3.
An object is placed 15cm in front of a convex mirror and an image is produced 5cm behind the mirror . Calculate the focal length of the mirror.
Solution
Using Real is Positive Convention
u = +15cm,
v = -5cm (-ve because a covex mirror always produces a virtual image)
Using the image Formula
1/u + 1/v = 1/f
1/15 + -1/5 = 1/f
1/15 - 1/5 = 1/f
1/f = -2/15
f = -7.5cm
Example 4.
An object is placed 10cm in front of a concave mirror whose radius of curvature is 12cm. Calculate the position, nature and magnification of the image produed. Use both Real-is-Positive and New Cartesian for the calculation.
Solution
(a) Using Real-is-Positive
r = 12cm
f = r/2 = 12/2 = 6cm
u = 10cm
Image Formula
1/u + 1/v = 1/f
1/v = 1/f - 1/u
1/v = 1/6 - 1/10
1/v = 1/15
v = 15cm
Since v is +ve, image is real
m = v/u
m = 15/10
m = 1.5
(b) Using New Cartesian
r = -12cm (front of the mirror will measure from left)
f = -12/2 = -6cm
u = -10cm (the same reason)
Image Formula
1/u + 1/v = 1/f
1/v = 1/f - 1/u
1/v = -1/6 - 1/10
1/v = 1/10 - 1/6
1/v = -1/15
v = -15cm
Image is 15cm from mirror on the same side as the object. So image is real.
m = v/u
m = -15/-10
m = 1.5
The same result were obtain whatever convention used
Rationale:
These topics encompass the fundamental concepts of light waves, their interactions with matter, and practical applications in daily life. Understanding these principles is essential in fields such as physics, optics, photography, and astronomy.Prerequisite/ Previous knowledge:
Production and propagation of wave.Learning Materials:
constructed pinhole camera, simple periscope, mirrors, etc.Reference Materials:
1. New School Physic for Senior Secondary Schools, by M. W. Anyakowa.2. Lamlas's SSCE and UTME, by O. Ajaja and H. B. Olaniyi.
3. Okeke P.N and etal (2011), Macminian, Senior Secondary School Physics, New Edition.
4. Farinde E.O and etal (2015), Essential Physics. 6th Edition,
Lesson Development:
| STAGE | TEACHER'S ACTIVITY | LEARNER'S ACTIVITY | LEARNING POINTS |
|---|---|---|---|
| STEP I: PREVIOUS KNOWLEDGE full class session |
The teacher asks learners questions based on previous knowledge. 1. What do you understand by wave motion? 2. Explain some basic descriptions of a wave. 3. Explain with examples types of Waves. 4. What are the properties of waves? |
Learners' response to teacher's question. 1. wave motion is a disturbance in a medium that carries energy without a net movement of particles. It may take the form of elastic deformation, a variation of pressure, electric or magnetic intensity, electric potential, or temperature 2. Some basic descriptors of a wave: (i) Wavelength is the distance between two successive identical parts of the wave. (ii) Amplitude is the maximum displacement from the neutral position. This represents the energy of the wave. Greater amplitude carries greater energy. (iii) Displacement is the position of a particular point in the medium as it moves as the wave passes. (iv) Maximum displacement is the amplitude of the wave (v) Frequency (Ζ) is the number of repetitions per second in Hz (vi) Period (T) is the time for one wavelength to pass a point. (vii) The velocity (v) of the wave is the speed at which a specific part of the wave passes a point. The speed of a light wave is c. 3. The types of waves are: (i) Transverse Waves: Waves in which the medium moves at right angles to the direction of the wave. Examples of transverse waves: β Water waves (ripples of gravity waves, not sound through water) β Light waves β S-wave earthquake waves β Stringed instruments β Torsion wave The high point of a transverse wave is a crest. The low part is a trough. (ii) Longitudinal Wave: A longitudinal wave has the movement of the particles in the medium in the same dimension as the direction of movement of the wave. Examples of longitudinal waves: β Sound waves β P-type earthquake waves β Compression wave Parts of longitudinal waves: Compression: where the particles are close together. Rarefaction: where the particles are spread apart. (iii) Mechanical waves: A wave which needs a medium in order to propagate itself. Sound waves, waves in a slinky, and water waves are all examples of this. (iv) Matter Waves: Any moving object can be described as a wave When a stone is dropped into a pond, the water is disturbed from its equilibrium positions as the wave passes; it returns to its equilibrium position after the wave has passed (v) Electromagnetic Waves: These waves are disturbance that does not need any object medium for propagation and can easily travel through the vacuum. They are produced due to various magnetic and electric fields. The periodic changes that take place in magnetic and electric fields and therefore known as electromagnetic waves. 4. The prime properties of waves are as follows: (a) Amplitude β Wave is an energy transport phenomenon. Amplitude is the height of the wave, usually measured in metres. It is directly related to the amount of energy carried by a wave. (b) Wavelength β The distance between identical points in the adjacent cycles of crests of a wave is called a wavelength. It is also measured in metres. (c) Period β The period of a wave is the time for a particle on a medium to make one complete vibrational cycle. As the period is time, hence is measured in units of time such as seconds or minutes. (d) Frequency β Frequency of a wave is the number of waves passing a point in a certain time. The unit of frequency is hertz (Hz) which is equal to one wave per second. The period is the reciprocal of the frequency and vice versa. (e) travelled per time of travel. The speed of a wave refers to the distance travelled by a given point on the wave (crest) in a given interval of time. |
Confirming previous knowledge. |
|
|
|||
| STEP II: INTRODUCTION full class session |
The teacher introduce the topic for discussion and probe learners to defined light. Imagine you're in a dark room. You aren't able to see anything. Your eyes need light to see. Even the smallest amount of light helps your eyes view the world. Did you know your eyes can detect the flame of a candle on a dark night about a mile away? You would think that the light from that candle moves in a straight line from the flame to your eyes, but a beam of light is actually an energy source that travels as a wavelength, and it moves quickly. Light is a form of energy called luminous energy. There are various sources of light namely the sun, stars, these are natural sources. Candles, electric torch, electric lamp, incandescent, arch lights and fluorescent light are called artificial sources of light. Light is an electromagnetic wave. It can pass through a vacuum and through a material medium. If light shines on a body,part of the light is transmitted through the body, the rest is reflected. The amount of light passing through a body depends on the nature of the body. If a large percentage of light falling on a body passes through it, the body is said to be transparent. Examples are glass and water. Some objects like frosted glass and tissue paper allow some small amount of light to pass through them. Such objects are called translucent objects. Some objects do not allow light to pass through them ,such objects are called Opaque objects. Shadow is produced by the obstruction of light by an opaque object. The shade under a tree or canopy,on bright sunny day is a shadow produced by the opaque tree or canopy. |
Learners listen to teacher and defind light as an electromagnetic wave with a wavelength in the visible part of the electromagnetic spectrum (380 to 700 nanometres). That is, light is any electromagnetic wave that we can see with our eyes. | Developing the idea of the topic light. |
| STEP III: EXPLORATION Sources of Light |
The teacher guides learners to list and explain sources of Light. | Learners' expected response, Light is a form of energy that helps us to see all the things around us. Light can come from different sources. Let's look at some of these sources now. There are countless sources of light, but they can all be categorized under either of the two following categories :- Natural sources Artificial sources 1. Natural Light Sources: The universe is filled with objects that emit light. Some light from these sources reaches the earth. The following things in nature have the ability to emit light: The Sun is the major source of light for the earth. The sun is a massive ball of fire, at the centre of which nuclear fusion produces massive energy. This energy comes out as heat and light. The light from the sun is one of the major factors behind the sustainability of life on earth. Every other star produces light too, but only a small or no amount of it reaches the earth because of the huge distance. The moon provides light as well but it cannot produce light on its own. The light that we get from the moon is the light reflected by it from the sun. Some living organisms have the ability to produce light too. It is called bioluminescence. It is the effect of certain chemical reactions within the organism. Fireflies, jellyfish, glow-worm, certain deep-sea plants, and microorganisms can be cited as examples. Certain other natural phenomena such as lightning and volcanic eruptions also emit light. 2. Artificial Light Sources: Apart from natural sources, light can be produced artificially too. The different light sources produced artificially can be put under three broad categories:- Incandescent Sources: When certain objects are heated to a high temperature, they begin to emit light. Both infrared and visible light is produced in the process. Example:- Candle, incandescent lamp. Luminescent Sources: Light can be produced by accelerating charges in a luminescent material. One common way of doing it is by passing current through the material. Example:- Fluorescent tube light, electric bulb Gas Discharge Sources: Passing electricity through certain gases at very low pressure can produce light too. Example β Neon lamp, Sodium lamp. |
Properties of Sources of Light |
|
|
|||
| STEP IV: DISCUSSION Transmission of light |
The teacher explain Transmission of Light with the studends and asked probing questions to help learners defined ray, beam and types of beam. The phenomenon of transmission of light takes place when a ray of light hits a transparent object that is translucent from where light can penetrate the material in order to travel its way through the object. As light moves as a wave, it is visible to the human eye when it is reflected and absorbed in real-time. The process of transmission takes place when the waves of light move wavy through a material that is eligible to absorb the light after it hits the object. The process of transmission depends on the transmittance of a material that is proportional to the phenomenon of light moving from one side to the other side In simple words, the process of transmission of light takes place when, light passes through some translucent or transparent object and gets refracted from it. |
Learners listen to teacher and make contribution. The light traveling in any one direction in a straight line is called a ray of light. A group of light rays given out from a source is called a beam of light. Different Types of Beams of Light: Beams of Light can be of 3 types. They are parallel, convergent and divergent. 1. Parallel: When Rays from a distant point source travel parallel to each other in a particular direction, it forms a parallel Light Beam. The sunRay is an example of a parallel Beam of Light. 2. Convergent: In a convergent Beam, the Light Rays from a source of Light, eventually meet or converge to a point. 3. Divergent: In a divergent Beam, the Light Rays disperse away from a source of Light. |
Transmission of light |
| The teacher explain the pin-hole cameral and guides students to explain natural effect of rectilinear propagation of light. PIN- HOLE CAMERA: A pinhole camera is a simple camera without a lens but with a tiny aperture (the so-called pinhole) effectively a light-proof box with a small hole in one side. Light from a scene passes through the aperture and projects an inverted image on the opposite side of the box, which is known as the camera obscura effect. The size of the images depends on the distance between the object and the pinhole. The pin-hole camera consists of a closed box or tin having a very tiny hole at the middle and a semi-transparent paper at one side of the box which produces an inverted, diminished (smaller) image of the object. Magnification is defined as the ratio of the image distance to the object distance or the ratio of the image height to the object height. |
Learners expected answer. 1. Sun as the point of source. 2. A shadow is an area which light rays from a source cannot be reach. A shadow is a dark area where light from a light source is blocked by an object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or a reverse projection of the object blocking the light. 3. Eclipse: The sun produces its own light, and therefore, it is a luminous object. The earth and the moon are non-luminous, object. The moon is seen by means of the sunlight reflected from the moonβs surface. E.g (a) Eclipse of the sun: This is experienced when the moon comes between the sun and the earth, and parts of the earth surface fall in the umbra and penumbra of the moonβs shadow. (b) Eclipse of the moon: This occurs when the earth comes directly between the sun and the moon. When the moon is on the opposite side of the earth, eclipse of the moon occur. (c) Annular eclipse of the sun: this occurs when the earth and the moon are in positions and the rays at the edge of the moon are intersected before reaching the earth. |
Rectilinear propagation of light | |
|
|
|||
| STEP V: APPLICATION Reflection on a Plane Mirror, Linear magnification and the mirror formula |
The teacher explain reflection of light on plane mirror with the students. A single image is formed when an object is placed in front of a mirror. What happens if we use two mirrors? Since reflective surfaces such as mirrors are very good at preserving the intensity of light in a reflection, a single light source can be reflected multiple times. These multiple reflections are possible until the intensity of light becomes low to the point that we cannot see. This means that we can have almost infinite multiple reflections. We can also see an image in every individual reflection. This means that each image is the result of an image or an image of an image. The number of images we see depends on the angle between the two mirrors. We see that as we go on decreasing the angle between the mirrors, the number of images increases. And when the angle becomes zero, i.e., when the mirrors become parallel, the number of images becomes infinite. This effect can be easily observed when your barber uses another smaller mirror to show you the back of your head. When this happens, not only do you see the back of your head, but you also see innumerable images of yourself. The variation of the number of images of an object placed between two mirrors with the angle between the mirrors can be described by a simple formula: Number of images = 360Β°/angle between mirrors. |
The students listen attentively and make contribution. When the light rays get stroked on the flat mirror, they get reflected back. According to the laws of reflection, the angle of reflection is equal to the angle of incidence. The image is obtained behind the plane, which is present in the mirror. This process of obtaining a mirror image which is virtual and erect is known as a reflection on a plane mirror. |
Reflection on a Plane Mirror Following are the characteristics of an image formed by the plane mirror: The image obtained by the plane mirror is always erect and virtual. The image size and the size of the object, both are equal. The distance between the image obtained and the mirror is the same as the distance at which the object is placed. Laterally inverted images are obtained. |
| The teacher explain reflection of light on curved surface with the students. We have already looked at reflection by plane mirrors. When the reflecting surface is instead curved, we call it a curved mirror. There are two types of curved mirrors; concave and convex mirror. Curved mirrors whose reflecting surfaces curve inwards are called concave mirrors while those whose reflecting surfaces bulge outwards are called convex mirrors. |
Learners make contribution Curved mirrors are parts of a sphere. The following terms are used in curved mirrors: β Pole (P): it is the centre of the mirror. β Centre of curvature (C): it is the centre of the sphere of which the mirror is part. β Radius of curvature (r): it is the radius of the sphere of which the mirror is part. β Principal axis: it is a line drawn through the pole of the mirror and the centre of curvature. β Principal focus (F): for a concave mirror, it is the point at which all rays parallel and close to the principal axis converge at after reflection. In the case of a convex mirror, it is the point at which all rays parallel and close to the principal axis appear to diverge from after reflection.(See the figure above). It is also called the focal point. β Focal plane: it is a plane perpendicular to the principal axis and passes through the focal point. It is the plane where parallel rays but not parallel to the principal axis converge at or appear to diverge from after reflection. β Focal length f: it is the distance between the pole of the mirror and its focal point. |
Reflection on a Curved Mirror When rays are produced behind the mirror, they are indicated using dotted lines. This means that they are imaginary or virtual. Hence the focal point and focal length of a concave mirror are real while the focal point and focal length of a convex mirror are virtual. A real focal length is given a positive sign while a negative focal length is given a negative sign. |
|
|
|
|||
| The teacher explain Linear magnification and the mirror formula. Linear magnification is defined as the ratio of the image size to the object size. Magnification = Image size / Object size. Similarly, it can be expressed as the ratio of the distance of the image to the distance v of object u from the mirror. Magnification has no unit. Suppose an object is placed u cm in front of a spherical mirror of focal length f such that the image is formed v cm from the mirror, then u, v and f are related by the equation. 1/f= 1/u + 1/v This equation is referred to as the mirror formula. The formula holds for both concave and convex mirrors. When applying the mirror formula, it is necessary to observe the following points: 1. That all distances are measured from the mirror as the origin. 2. All real distances are positive while all virtual distances are negative. 3. A concave mirror has a positive focal length while a convex mirror has a negative focal length. |
The teacher guides learners to use the formulas to solve problems 1. Determine the position, size and nature of the image of an object 4cm tall placed on the principal axis of a concave mirror of focal length 15cm at a distance 30cm from the mirror. Solution u = 30cm, f = 15cm, ho = 4cm 1/f = 1/u - 1/v 1/v = 1/f - 1/u i/v = = 1/15 β 1/30 1/v = 1/30 v = 30cm Also, m = v/u = hi/ho Thus, hi = (30cm x 4cm)/30cm hi = 4cm. Thus the image formed is real and same size as the object. 2. A convex mirror of focal length 9cm produces an image on its axis 6cm from the mirror. Determine the position of the object.SOLUTION f = -9cm, v = -6cm. 1/f = 1/u - 1/v 1/u = 1/f β 1/v 1/u = -1/9 β (-1/6) 1/u = (-2+3)/18 1/u = 1/18 u = 18cm |
Application of Linear magnification and the mirror formula to solve problems. | |
| STEP VI: EVALUATION | The teacher asks the learners questions. 1. What is light, and the sources of light? 2. What are the properties of light? 3. Explain transmission of light and list types of beams. 4. What is a ray box? |
The learners answer the teacher's questions. 1. Light is a wave, it is visible form of energy. The light from the sun comes to us by radiation. Other sources of light are; A. Luminous object: These are object that produce their own light e.g the Sun, Stars, fire flies, Lamp, Candles, and Electric bulbs B. Non- luminous objects: These are objects that do not produce their own light; they are only seen when light from sources fail on them and is reflected into our eyes. 2. (a) Light travels in a straight line. (b) Light produces shadow when obstructed along its path of propagation. (c) Light can be reflected. (d) Light can be refracted. (e) Light can be diffracted. (f) Light can be polarized. 3. Transmission of light, Light is an electromagnetic wave. It can pass through a vacuum and through a material medium. The direction or path along which the light travels is called a light ray or beam. TYPES OF BEAMS LIGTH. 1. Convergence beams. 2. Divergence beams. 3. Parallel beams. 3. Conditions necessary for total internal reflection - The wave must be traveling from a denser medium to a less dense medium - The angle of incidence of the wave in the denser medium must be greater than the critical angle for the medium. 4.Ray box: Rays of light are produced in the laboratory by means of a ray box. It consists of a box made of wood or cardboard inside which is a source of light. E.g candle or an electric lamp. |
Ask the learners questions to assess the achievement of the set objectives. |
| CONCLUSION | The teacher concluded the lesson with the students with question. Applications of plane and curved mirrors are as follows: plane mirrors are used in periscope and kaleidoscope, while curved mirrors are used in automobiles, torch lights, looking glasses, security doors and places, in shaving and by dentists etc. Concave is a spherical mirror which has a reflecting surface dented inwards. Concave mirror reflect and focus incoming light rays (parallel) at a point called the focus point. Convex is a curved mirror where the reflective surface bulges out toward the light surface. This bulging out surface reflects light outwards and is not used to focus light. |
Learners listen to the teacher and solve the question. An object is placed 15cm in front of a convex mirror and an image is produced 5cm behind the mirror. Calculate the focal length of the mirror. SOLUTION using the mirror formula, 1/f = 1/u + 1/v 1/f = 1/15 + 1/-5 1/f = 1/15 - 1/5 1/f = -2/15 f = -7.5cm |
Application of plane and curved mirrors. |
|
|
|||
| ASSIGNMENT | The teacher gives learners a take home. | THEORY 1. An object of height 40cm is placed 0.8m in front of a pin-hole camera of length 16cm. What is the magnification and height of image produced? 2. (a) What do you understand by the term lateral inversion? (b) write your first name in block form to buttress (a) 3. Differentiate between concave and convex mirror 4. Two plane mirrors inclining at an unknown angle, forms 11 images. Find the value of the angle. 5. Mention three uses of plane mirrors 6. (a) Give the differences between real and a virtual image. (b) A magnified, virtual image is formed 12cm from a concave mirror of focal length 18cm. calculate the position of the object and the magnification of the image. 7. (a) Explain with the aid of diagram how the image of an object is formed by a plane mirror. (b) State four characteristics of the image. 8. (a) Define the following terms (i) principal focus (ii) radius of curvature (iii) principal focus. (b) The screen of a pinhole camera is a square of side 160mm and it is 150mm behind the pole. The camera is placed 11m from a flag staff and positioned so that the image of the flag staff is formed centrally on the screen. The image occupies three-quarters of the screen. What is the length of the staff? OBJECTIVE 1. Which of the following abatement is true of virtual image (a) it is formed on the screen (b) it is formed by the intersection of actual rays (c) rays of light do not pass through it (d) all of the above (e) none of the above 2. An object is placed between two plane mirrors inclined at 600 to each other. How many images will the observer see? (a) 6 (b) 5 (c) 4 (d) 3 (e) 2 3. An object is place 15cm in front of a concave mirror of focal length 20cm, the image formed is (a) real, inverted and diminished (b) real, inverted and magnified (c) virtual, erect and diminished (d) virtual, erect and magnified (e) virtual, inverted and magnified 4. A concave mirror can be used to produce can be used to produce a parallel beam of light if a light bulb is placed (a) between its focus and the pole (b) at its focus (c) at its center of curvature (d) between its focus and the center of curvature (e) none of the above 5. When an image is formed in a plane mirror, the image formed will be (a) the same size as the object (b) smaller than the object (c) laterally inverted (d) always virtual (e) all of the above 6. Using the real is positive sign convention determine the sign of the focal length of a convex mirror (a) positive (b) negative (c) neutral (d) none of the above (e) options (a) and (b) 7. An object is placed in front of a concave mirror of radius of curvature 12cm. if the height of the real image formed is three times that of the object, calculate the distance of the object from the mirror (a) 24 cm (b) 16 cm (c) 12 cm (d) 8 cm (e) 4 cm 8. A magnified erect image four times the size of the object is formed by a concave mirror of focal length 12cm. what is the distance of the image from the pole of the mirror? (a) -36cm (b) -18cm (c) -24cm (d) -3.6cm (e) 24cm 9. A boy walks away from a plane mirror at a constant speed of 5.0 m/s in a direction normal to the surface of the mirror. At what speed does his image move away from him? (a) 5.0 m/s (b) 2.50 m/s (c) 3.5.0 m/s (d) 1.25.0 m/s (e) 0.00 m/s 10. The image of an object is located 6cm behind a convex mirror. if its magnification is 0.6, calculate the focal length of the mirror (a) 3.75 cm (b) 6.60 cm (c) 10.00 cm (d) 15.00 cm (e) 20.00 cm |
Improving their level of understanding on Light waves. |