Application of Light waves
Optical Instrument
Subject: Physics
Theme: Waves-Motion Without Material Transfer
Topic: Application of Light waves
Sub Topic: Optical Instrument
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 Optical instruments
Optical instruments are any equipment that uses a single or several lenses to take in light as an input and perform any action on it, depending on the equipment's purpose, to either enhance an image for clearer viewing or enlarge it.2. List and Explain The Applications of Light Waves
Applications of light waves or types of optical instrument are simple camera, film projector, the human eye, simple microscope, compound microscope, Astronomical telescopes and prism binoculars.(i) Simple Camera
Simple camera consists of a light proof box with a converging lens in front and a light sensitive film at the back. There is a provision for adjusting the distance between the lens and the film, so that objects in front of the lens can always be focussed on the film by the converging lens. To obtain a better image the converging lens is sually replaced by a system of lenses.A shutter of variable speed, between the lens and the film admits or shuts off light from the film, and a diaphragm regulates the size of the an aperture which controls the amount of light energy sensitising the film.
A digital camera does not use films, however. The image, in this case, is captured by image sensors and reproduced in the many numbers of ways available.
(ii) Film Projector
A projector is an instrument used to produce a large image of a small object. The slide, or film, is placed behind the projector lens outside its focal length and is illuminated by a small but powerful source of light.The amount of light from the source actually going through the transparent slide or film is increased by using a condenser and a concave mirror.
The slide must be put into the projector upside down to give an image on the screen which is the right way up.
Below are the essential parts of a projector and their optical arrangements.
(a) The small but powerful light source to illuminate the object which is usually non-luminous. A converging mirror placed behind the light source helps to direct the light onto the film or slide.
(b) The condenser consisting of two piano. convex lenses collects the light from the light source and concentrates it on the slide or transparency and illuminates every pan of it strongly and evenly. Between the two lenses is usually an infra-red filter to prevent the passage of heat to the slide which might otherwise melt it.
(c) The slide carrier is a framework in which the slide is placed upside down, so that it will appear erect on the screen.
(d) The projection lens placed near the slide produces a real enlarged and inverted image of the slide and focuses it on to the screen. The lens is at a distance of between f and 2f from the object, where f is the focal length of the lens.
(e) The screen, usually white, receives the image which is the right way up if the object is inserted upside down into the slide carrier. The principle of the slide projector is uscd in tine-projectors, photographic enlargers and several other devices.
(iii) Astronomical Telescopes
Astronomical telescopes are used to magnify distant objects in order to see them clearly.There are three types of Astronomical telescopes:
(a) Refracting telescopes: These use a combination of lenses to form a magnified image.(b) Reflecting telescopes: These instruments use a combination of mirrors to form a magnified image.
(c) Catadioptric telescopes: These instruments use a combination of both lenses and mirrors to form images.
Telescopes are meant for viewing distant objects, producing an image that is larger than the image that can be seen with the unaided eye. Telescopes gather far more light than the eye, allowing dim objects to be observed with greater magnification and better resolution.
Galileo was the first to study the heavens and made monumental discoveries using a telescope. Among these are the moons of Jupiter, the craters and mountains on the Moon, the details of sunspots, and the fact that the Milky Way is composed of vast numbers of individual stars.
(iv) Microscopes
Microscopes are instruments designed to magnify small objects. A simple microscope uses only one lens for the sake of magnification whereas a compound microscope uses more than one lens. A compound microscope uses two lenses, one as the objective and the other as the eyepiece.(v) Prism Binoculars
Prism Binoculars are essentially a pair of astronomical telescopes each of which is fitted with two right angled isosceles prisms. The light passing through the objective is reflected from one prism to another prism and then emerges through the eyepiece.The function of the prisms is thus to re-invert the image formed by the objective and present an erect image of the object to the eyepiece which subsequently magnifies it. An erect and magnified image is thus presented to the viewer.
The prism binoculars as well as presenting an upright image are also very compact since they reduce the optical path of the light ray from the objective to the eyepiece, very appreciably. Also because the binoculars are usually made in pairs they have an additional advantage of bringing the rays from two telescopes inwards to match the distance apart of the two eyes.
(vi) The Human Eye
The optical system of the eye consist of the cornea, the aqueous, the vitreous humour and the lens. They form a real and inverted image of an external object on the retina. The retina transmits the impression created on it by the image through the optic nerve to the brain. The brain then interpretes the impression. The amount of light entering the eye through the pupil is regulated by the iris.Accommodation is the ability of the eye to alter or adjust the focal length of its lens so as to form clear images of objects at different distances on its retina. This adjustment is brought about through the action of the ciliary muscles.
Since the eyes are spaced slightly apart, each views an external object from a slightly different position, hence the images of the object falling on the retina are not exactly the same. The right eye sees more of the right side of the object, while the left eye sees more of the left side of the object.
Binocular vision is the combination of the two images formed by both eyes which gives an impression of depth and solidity and makes it Possible for us to see things in relief. such proper perspective of an object is impossible with one eye.
Like the camera the eye has a refracting system containing a converging lens which forms an image of an object in front of the eye on a light sensitive layer at the back of the eye. The essential parts of the eye and their various functions are:
(a) The Cornea: The thick transparent bulge in the front part of eye. It serves as a protective covering in front of the lens and allows light into the eye. It also partly focuses the light entering the eye.
(b) The Aqueous humour: This is the transparent liquid between the cornea and the lens and the vitreous humour is a jelly-like liquid between the lens and the rest of the eyeball. These liquids serve mainly to keep the eye in its spherical shape.
(c) The Iris: The Iris is behind the cornea. The iris is the part which gives the colour of the eye. It acts as a stop or diaphram of variable size. it has a tiny opening at its centre called the pupil which regulates the quantity of light entering the eye. The pupil looks black because the inner layer of the eye, the choroid, is black.
(d) The Crystalline Lens: The lens focuses the light from the object on to the light sensitive retina.
(e) The Ciliary muscles: The Ciliary muscles support the lens and attach it to the wall of eye. These muscles by their contraction and expansion, alter the focal length of the lens changing its shape.
(f) The Retina: The Retina is the light sensitive area of cells at the back of the eye. The image is formed on the retina which consists of light sensitive nerve endings which are connected to the optic nerve. The optic nerve conveys the sensation a sight to the brain.
(g) The Yellow spot: The Yellow spot is the most sensitive spot of the retina. It is here that light entering the eye are usually brought to a focus and here the clearest image is formed.
(h) The Blind spot: The Blind spot is the point of exit of the optic nerve from the retina. It is insensitive to light.
(i) Sclerotic layer: This is the outer covering of at eye.
Mechanism of vision
The organ of vision is the eye. The eye can only see when rays of light comes from the object to the eye. An image is seen by the eye through the following steps.(i) Light from the objects falls on the cornea which partly focuses the light rays to enter the eye through a tiny hole called pupil. The pupil also controls the amount of light entering the eye.
(ii) The light rays falls on the lens which refracts the rays to form an image of the objects on the light sensitive part of the eye called the Retina. The most sensitive part of the retina is the yellow spot and it is here that the sharpest image is formed.
(iii) There are many light sensitive nerves attached to the retina. This send messages of the image formed to the brain which then interprets the image for the person to understand. images formed on retina are upside down (inverted) but the brain flips them back to produce perfect picture.
Persistence of Vision
When light from an object falls on the retina, it excites it and the clear image formed on the retina leave a sensation of Vision which persists for a short but definite length of time even after the light is removed. This retention of the vision of the image is known as persistence of vision.3. Normal Vision and the Defects of Vision
Vision is the ability to see things physically.
Near Point and Far Point
(i) Near Point: the nearest point at which an object can be clearly seen is called the near point.
For people with normal vision, the nearest distance at which an object can be clearly seen is called the near point, it is about 2S cm from the eye. This is the limit to which the ciliary muscles can operate, for at this point they are strained to a maximum, and the accommodation is, therefore the greatest. The distance from the near point to the eye is known as the least distance of distinct vision.
(ii) Far Point: the farthest point at which an object can be clearly seen is called the far point.
For the normal eye, the far point is at infinity. At this point the ciliary muscles are most relaxed and the lens is slim.
People whose eyes are not functioning normally are said to have some defects of vision. Two of the most common types are the long sight and the short sight. Both defect arise when the eye lens is unable to accommondate effectively.
1. Long Sight (hypermetropia)
A long sighted person can see object at infinity (far object) but cannot see near objects clearly. His near point is farther away than 25cm, the normal near point. An object at the normal near point N cannot be clearly seen by this eye because the rays from the object are brought to a focus at a certain distance behind the retina. this is caused by the eyeball being too short due to advancing age or the crystalline lens not being sufficiently convergent.Correction
The defect is corrected with the aid of a suitable converging spectacle lens placed I front of the eye. The converging lens reduces the divergence of the rays entering the eye from an object just sufficiently to make the rays appear as if they originated from O, the near point of the defective eye. Hence a clear image of the object is formed on the retina.
2. Short Sight (myopia)
A short sighted person cannot see distant objects clearly. This is because his eyeballs are too long or the crystalline lens is too converging that rays from distant objects are brought to a focus in front of the retina instead of at the retina. The far point at O is thus less than the normal far point which is at infinity. Light rays from his far point are brought to focus at the retina. Therefore, objects at O can be clearly seen, by the defective eye.Correction
Short sight can be corrected by placing a suitable concave or diverging spectacle lens in front of the eye. The action of this lens is to diverge light rays entering the eye from a distant object and make them appear to be coming from the person's far point.
4. Solve problems on eye
Example 1Find the focal length of the lens required to correct the defect of Vision of a man whose near point is 50cm. Assume the least distance of Vision of the normal eye to be 25cm.
Solution
For this man to see objects clearly at 25 cm from the eye, the image must appear to be at 50 cm.
Hence;
u = 25cm,
v = -50cm (the final image is virtual)
Using Lens formula
1/v + 1/u = 1/f
1/f = 1/v + 1/u
1/f = 1/-50 + 1/25
1/f = 1/25 - 1/50
1/f = 1/50
f = 50cm
Hence, the man needs a convex lens of focal length of 50 cm.
A short sighted man cannot see clearly objects beyond 90 cm from his eye. How can this defect be corrected?
Solution
The defect of short sight is corrected by the use of diverging lens. An object at infinity must be made to appear to be at his far point which is 90 cm away. The image of the distant object must be formed on the same side of the lens as the object
Hence;
u = ∞ (infinity),
v = -90cm
Using Lens formula
1/f = 1/v + 1/u
1/f = 1/-90 +1/∞
Since 1/∞ = 0
1/f = 1/-90
f = -90cm
Therefore, a concave lens of focal length 90cm is required
5. List and Explain Comparison of the Human Eye and the Camera
similarities between the human eye and the camera
(i) The camera includes light-tight box painted black inside. The human eye is impregnated with black pigments within.(ii) The pupil in the eye serves the same purpose as the aperture of the camera.
(iii) Both the eye and the camera have converging lens to focus rays from an external object.
(iv) Both have light sensitive material on which the rays are focused, the retina in the eye and the film in the camera.
(v) The iris in the eye serves the same function as the diaphram in the camera, to regulate the amount of light entering the equipment.
Differences between the human eye and the camera
(i) The Human eye has the power of accommodation, the ability of the lens to focus points at different distances on the retina by altering the focal length of the lens. Hence the eye lens has a variable focal length but the focal length of the camera lens is fixed.(ii) The distance between the lens and the retina is fixed in the human eye. In the camera the distance between the lens and film can be varied.
(iii) The eye can suffer from defects of vision unlike the camera.
(iv) The eye is a biological organ while the camera is a mechanical device, leading to other differences.
Rationale:
Light is the only electromagnetic radiation that is visible to the human eye. It is made of little packets of energy known as photons. When a light wave is an incident on an object, a number of things can happen. It is either absorbed, transmitted or reflected. The manner in which visible light interacts with an object depends on the frequency of light and the nature of the atoms of the object.Prerequisite/ Previous knowledge:
Light wave, simple camera, film projector, the human eye,simple microscope, compound microscope, telescope and prism binoculars.Learning Materials:
simple camera, film projector, the human eye,simple microscope, compound microscope, telescope and prism binoculars.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,
5. New System Physics by Dr. Charles Chow.
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. Define the principal focus of a concave mirror. 2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length? 3. Name the mirror that can give an erect and enlarged image of an object. 4. Why do we prefer a convex mirror as a rear-view mirror in vehicles? 5. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why? 6. Light enters from air to glass, having a refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 108 ms-1. 7. Find the power of a concave lens of focal length 2 m. 8. Name the type of mirror used in the following situations. (a) Headlights of a car (b) Side/rear-view mirror of a vehicle (c) Solar furnace Support your answer with a reason. |
Learners' response to teacher's question. 1. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is called the principal focus of the concave mirror. 2. Radius of curvature (R) = 20 cm Radius of curvature of the spherical mirror = 2 × Focal length (f) R = 2f f= R/2 Therefore, the focal length of the spherical mirror is 10 cm. 3. The mirror that can give an erect and enlarged image of an object is a Concave Mirror. 4. A convex mirror is preferred as a rear-view mirror in cars and vehicles as it gives a wider field of view, which helps the driver see most of the traffic behind him. Convex mirrors always form an erect, virtual, and diminished image of the objects placed in front of it. 5. The light ray bends towards the normal. When a light ray enters from an optically rarer medium (which has a low refractive index) to an optically denser medium (which has a high refractive index), its speed slows down and bends towards the normal. As water is optically denser than air, a ray of light entering from air into water will bend towards the normal. 6. Refractive index of a medium (nm) = Speed of light in vacuum/Speed of light in the medium Speed of light in vacuum (c) = 3 × 108 m/s Refractive index of glass (ng) = 1.50 Speed of light in the glass (v) = Speed of light in vacuum/ Refractive index of glass = c/ng =3 × 108/1.50 = 2x 108 ms-1. 7. The focal length of the concave lens (f) = 2 m Power of lens (P) = 1/f 8. (a) Concave Mirror: Concave mirrors can produce a powerful parallel beam of light when the light source is placed at their principal focus. (b) Convex Mirror: Because of its largest field of view. (c) Concave Mirror: Because it concentrates the parallel rays of the sun at a principal focus. |
Confirming previous knowledge. |
| STEP II: INTRODUCTION full class session |
The teacher introduce the topic for discussion. Applications of light waves are simple camera, film projector, the human eye,simple microscope, compound microscope, telescope and prism binoculars. Simple camera consists of a light proof box with a converging lens in front and a light sensitive film at the back. Projector is an instrument for projecting on a screen an enlarged image of a transparent object. For example a slide. The Human Eye is a pair located in the eye sockets of the brain which is held in position by some muscles that connect the blood vessels with nerve cells. Four functions of human eye are as follows: 1. the cornea serves as a protective covering in front of the lens and allows light into the eye. 2. the retina is the sensitive area of cells at the back of the eye. The image is formed on the retina. 3. the iris is behind the cornea, it is the part which gives colour of the eye. It acts as a stop or diaphragm of variable size. 4. the blind spot is the point of exit of the optic nerve from the retina and it is sensitive to light. Accommodation is the ability of the eye to alter or adjust the focal length of its lens so as to form clear images of objects at different distances on its retina. This adjustment is brought about through the action of the ciliary muscles. |
Learners listen to teacher, ask questions and make contribution. | Developing the idea of the topic, Applications of light waves. |
|
|
|||
| STEP III: EXPLORATION Grouping |
The teacher presents the instructional material to the students and divides them into four groups. The representative of a group is asked to discuss the uses of the material. Aim is to show Applications of light waves. | Learners' expected answer The students carry out the instructions accordingly. Simple microscope has a single converging lens of short focal length while compound microscope consists of two converging lenses of short focal length. The eyepiece has a longer focal length than the objective. Astronomical telescope is used in viewing distant objects, such as the stars and the planets. its consists of a converging objective lens of long focal length and a converging eyepiece of short focal length. The Galilean telescope uses a convex lens of long focal length as the objective and a concave lens of short focal length as the eyepiece. The prism binoculars are in reality a pair of twin telescopes mounted side by side, one for each of the two eyes. Each binocular consists of two converging lenses and a pair of total reflecting prism which invert the rays to give erect images. |
Applications of light waves. |
| STEP IV: DISCUSSION | The teacher guides the students as follow: 1. The simple lens camera: the simple lens camera consists of a light-proof box with a converging lens in front and a light sensitive film at the back. The functions of the lens are to focus images of the object to be photographed onto the light sensitive plate of film. The simple lens camera has a fixed distance between the lens and the film. This distance is equal to the focal length of the Lens. Some cameras have bellows or focusing rings by which the distance between the lens and the film can be adjusted. 2. The Projector: the projector is an instrument for projecting on a screen an enlarged image of a transparent object e.g. a slide. The principle of the slide projector is used in cine-projectors, photographic enlarges and several other devices. 3. The Human Eye: The Human Eye is a pair located in the eye sockets of the brain which is held in position by some muscles that connect the blood vessels with nerve cells. The eye is made of three layers; (i) Scheroid layer (ii) Choroids layer (iii) Rectina layer The scheroid and choroids layers are the outer and sub-inner layers respectively, while the retina layer is the inner-most layer, the most sensitive part of the eye is called fovea. The essential parts of the eye and their various functions are: The cornea: this is the thick transparent bulge in the front part of the eye. It serves as a protective converging in front of the Lens and allows light into the eye. The aqueous humour: is the transparent liquid between the cornea and the Lens and the vitreous humour is a jelly-like liquid between the lens and the rest of the eyeball. The iris: is behind the cornea. The iris is the part which gives the colour of the eye. It acts as stop or diaphragm of variable size. The crystalline lens: which focuses the light from the object on to the light sensitive retina. The retina: is the light sensitive area of the cells at the back of the eye. The ciliary muscles: which support the Lens and attach it to the wall of eye. The human eye, like the camera, the eye has a refracting system containing a converging lens which forms an image of an object in front of the eye on a light sensitive layer at the back of the eye. The optical system of the eye consists of the cornea, the lens, and the aqueous and vitreous humour. They operate to form a real and inverted image of an external object on the retina. The retina transmits the impression created on it by this image to the brain through the optic nerve. The brain then interprets the impressions. |
The students listen, contribute and ask question | Applications of light waves. |
| STEP V: APPLICATION | The teacher guides learners to explain persistence of vision and Normal vision and the Defects of Vision | The students explain persistence of vision. When light from an object falls on the retina, it excites it and the clear image formed on the retina leave a sensation of Vision which persists for a short but definite length of time even after the light is removed. This retention of the vision of the image is known as persistence of vision. Near Point: the nearest point at which an object can be clearly seen is called the near point. For a normal eye, the near point is about 25cm from the eye. This is the limit to which the ciliary muscles can operate, for at this point they are strained to a maximum, and the accommodation is, therefore the greatest. The distance from the near point to the eye is known as the least distance of distinct vision. Far Point: the farthest point at which an object can be clearly seen is called the far point. For the normal eye, the far point is at infinity. At this point the ciliary muscles are most relaxed and the lens is slim. The accommodation is also the least. Long Sight (hypermetropia): A long sighted person can see far object but cannot see near objects clearly. His near point is farther away than 25cm, the normal near point. An object at the normal near point N cannot be clearly seen by this eye because the rays from the object are brought to a focus at a certain distance behind the retina. this is caused by the eyeball being too short or the crystalline lens not being sufficiently convergent. It's Correction The defect is corrected with the aid of a suitable converging lens placed I front of the eye. The converging lens reduces the divergence of the rays entering the eye from an object just sufficiently to make the rays appear as if they originated from B, the near point of the defective eye. Hence a clear image of the object is formed on the retina. Short Sight (myopia) A short sighted person cannot see distant objects clearly. This is because his eyeballs are too long or the crystalline lens is too converging that rays from distant objects are brought to a focus in front of the retina instead of at the retina. The far point at A is thus less than the normal far point which is at infinity. Light rays from his far point are brought to focus at the retina. Therefore, objects at A can be clearly seen, by the defective eye. It's Correction Short sight can be corrected by placing a suitable concave or diverging lens in front of the eye. The action of this lens is to diverge light rays entering the eye from a distant object and make them appear to be coming from the person's far point. |
Persistence of Vision |
|
|
|||
| STEP VI: EVALUATION | The teacher asks the learners questions. List and explain two optical instrument |
The learners expected answers. 1. Compound Microscope: The compound microscope produces a greater magnification than the simple microscope. It has two lenses, the objective lens which has a short focal length and the eye piece used as the magnifying glass to view an image formed by the objective lens. The image formed by the objective lens is within the principal focus of the piece. So a final image is formed at the least distance of distinctive vision from the eye. 2. Astronomical Teloscope: An astronomical telescope is used for viewing distance objects like stars and planets. The astronomical telescope uses two convex lenses; the objective lens and the eye piece. The objective lens has a long focal length and forms a real image of a distant object at its focal plane. The position of the eyepiece and the objective lens must coincide along the principal focus so that the final image is at infinity. The astronomical telescope gives an inverted image which can be tolerated when looking at the stars but is at a disadvantage on the earth. |
Ask the learners questions to assess the achievement of the set objectives. |
| CONCLUSION | The teacher concluded the lesson with the students. LONG SIGHT (HYPERMETROPIA) A long sighted person can see objects at a distance but cannot see close objects clearly. His near point is more than 25cm which is the near point of the normal eye. It is caused by the eye ball being too short so that rays from object at 25cm from the eye are brought to focus behind the retina. It is corrected by converging lens placed in front of the eye for near vision. SHORT SIGHT (MYOPIA) A short sighted person cannot see distant objects clearly as rays from such objects are focused in front of the retina. His far point is less than the normal far point which is at infinity. It is corrected by the use of diverging lens. The diverging lens makes the object at infinity to appear to be at the person's far point. |
The students listen and make contribute | Defects of Vision. |
|
|
|||
| ASSIGNMENT | The teacher gives learners a take home. | OBJECTIVES 1. For correcting long sight defects in the human eyes, we require _______ (a) Converging lens (b) diverging lens (c) microscope (d) periscope. 2. Which of the following optical instruments does not make use of a lens? (a) projector (b)periscope (c ) eye (d) microscope. 3. The ability of the eye to focus object at different distances is called _______ (a) Power (b) accommodations (c) normal vision (d) long sight 4. Binocular vision (a) Restricts the field of view (b) Enables a person to see further (c ) Enables objects to be seen in relief (d)Enables objects to be seen clearly. 5. _______ Waves are set up in pipes (a) Stationary (b) longitudinal (c) transverse (d) electromagnetic. THEORY 1. State three types of eye defect and the types of lenses for correction. 2. State three types of optical instruments. |
Improving their level of understanding on optical instrument. |