NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
CHAPTER 12
'LIGHT'
KEYWORDS
Normal: A line making an angle of 90° to the line representing the mirror at the point where the incident ray strikes the mirror.
Angle of incidence: Angle between normal and incident ray.
Angle of reflection : Angle between normal and reflected ray.
Diffused reflection: When all the reflected rays form a plane surface are not parallel.
Regular reflection: Reflection from a smooth surface.
Dispersion: Splitting of light into its colours.
Blind spot: A spot in eye where no vision is possible.
Iris: A dark muscular structure behind the cornea.
Pupil: In iris, it is a small opening.
TEXT BOOK QUESTIONS
Q.1. Suppose you are in a dark room. Can you see object in the room? Can you see objects outside the room? Explain.
Ans. No, we cannot see any object placed, inside a dark room, because there is no reflection of light from object. If any object is outside the room, then we can observe the object, because in this case our eye will perceive a reflected light from outside object.
Q.2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the lens of reflection?
Ans:-
Regular Reflection
2. Reflected rays are parallel to 2. Reflected rays are not each other.
Different reflection
Q.3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) A polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) A mirror
(f) piece of paper
Ans. (a) A polished wooden table: Regular reflection - as, it is smooth surface.
(b) Chalk powder : Diffused reflection - as, it has irregular surface.
(c) Cardboard surface : Diffused reflection - as, it has irregular surface.
(d) Marble floor with water spread over it: Regular reflection - as it has regular & smooth surface.
(e) A mirror: Regular reflection - as, it has regular & smooth surface.
(f)A piece of paper : Diffused reflection - as, it has surface with irregularities
Q.4. State of laws of reflection.
Ans. Two laws of reflection are:
(a) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface lie in the same plane.
(b) The angle of incidence is equal to the angle of reflection.
Q.5. Describe an activity to show that the incident rays the reflected ray, the normal at the point of incidence lie in the same plane.
Ans. Materials required: White sheet of paper, a drawing board, drawing pins, a comb.
Procedure :
(i) A white sheet of paper on a drawing board is fixed.
(ii) A strip of plane mirror is taken and placed in the path of light ray.
(iii) The ray of light will be reflected.
(iv) Angle of incidence and angle of reflection on the sheet of paper is drawn.
(v) The incident and reflected rays along the paper is observed.
(vi) Now the sheet of paper is bent a little at the position of the reflected ray.
(vii) Keeping the incident ray and the normal in the plane of the drawing board, the reflected ray is tried to be seen now. The paper is put back and observed again and the activity is repeated for different angles of incidence.
Observation Firstly, when the paper is bend, then incident ray and normal ray are found to be on the plane of the paper. When paper is placed on the drawing board again, then one can see that the incident ray, reflected ray and the normal ray to the point of incidence all lie in the same plane.
Q.6. Fill in the blanks in the following:
(a) A person Im in front of a mirror seems to be...........from his image.
(b) If you touch your.........ear with right hand in front of a plane mirror it will be seen that your right ear is touched with................. in the mirror.
(c) The muscles attached to the eye lens make it thicker to see................objects
(d) Night birds have.............comes than rods in their eyes
Ans.
(a) -2m;
(b) - left, left;
(c) - near;
(d) - only a few
Q.Choose the correct option in the question 7 and 8
Q.7. Angle of incidence is equal to the angle of reflection-
(i) Always
(ii) Sometimes
(iii) Under special conditions
(iv) Never
Ans. Angle of incidence is equal to angle of reflection always.
Q.8. Image formed by a plane mirror is-
(i) virtual, behind the mirror and enlarged.
(ii) virtual, behind the mirror and of the same size as the object.
(iii) real at the surface of the mirror and enlarged.
Ans.
(iv) real, behind the mirror and of the same size as the object. Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.
Q.9. Describe the construction of kaleidoscope.
Ans. Three rectangular mirror strips of dimensions 15cm x 4cm (lxb) are joined together to form a prism. This prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer that the prism. This circular tube is now closed at one end with a cardboard disc. This disc has a hole in it through which we can see. At the other end of the circular tube; a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move
Q.10. Draw a labelled sketch of human eye.
Q.11. Gurmeet wanted to perform Activity 12.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher's advise?
Ans. As in laser light, only monochromatic rays are produced i.e. produced rays have only one colour that is why teacher advised Gurmeet not to use laser light.
Q.12.Explain how can you take care of your eyes?
Ans. It is necessary that you take proper care of your eyes.
(1) If there is any problem you should go to an eyes specialist. Have a regular checkup.
(ii) If advised, use suitable spectacles.
(iii) Too little or too much light is bad for the eyes. Insufficient light causes eyestrain and headache. Too much light, like that of the sun, or powerful lamps, can injure retina, which is very delicate.
(iv) Do not look at the sun or a powerful light, directly.
(v) Never rub your eyes if any small particle or dust goes into your eyes. Wash your eyes with clean water. If condition does not improve, go to a doctor.
(vi) Wash your eyes frequently with clean water.
(vii) Always read at the normal distance for vision. Do not read by bringing your book too close or keeping it too far.
Q.13. What is the angle of incidence of a ray if the incident and reflected rays are at an angle of 90°?
Ans. Let angle of incidence is i and angle of reflection is r, then
〈i+〈r = 90° (from question)
We have 〈i = 〈r From laws reflection
So 2〈i = 90°
So 〈i= 90°/2
=45ο Ans
Q.14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Ans. Number of images formed by two mirrors at an angle Ó¨, is n = 366°/θ
For parallel mirrors, Ó¨ = 0°
So. n=360/θ-1=∝-1=∝(infinite)
So, infinite images will be formed by parallel mirrors.
Q.15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in the figure. Draw the reflected ray from the second mirror.
Q.16. Yasir stands at A just on the side of a plane mirror as shown in figure. Can he see himself in the mirror? Also can he see image of objects situated at P, Q and R?
Ans. Yes, Yasir can observe his image as well as images of P, Q, R.
Q.17. (a) Find out the position of the image in the plane mirror of an object situated at A.
(b) Can Saba at B see this image?
(c) Can Yasir at C see this image?
(d) When Saba moves from B to C, Where does the image of A move?
The image of A is at same distance behind the mirror, is virtual and as same size of object A.
(b) Yes, Saba at B can see the image.
(c) Yes, Yasir at C can see the image.
(d) The image of A will remain stationary, as it does not depend on the motion of observer.
ADDITIONAL SHORT ANSWER TYPE
Q.1. How do we see objects?
Ans. When light from a light source falls on any object, it is scattered by it. The scattered light enters our eyes to form the image of the object and the object is, thus, seen.
Q.2. Define the following terms:
(i) Illuminated objects
(ii) Luminous objects
Ans. (i) Illuminated objects: The objects, which shine in the light of other objects are called illuminated objects.
(ii) Luminous objects: The objects, which emits their own light are called luminious objects.
Q.3. What is reflection?
Ans. When a ray of light falls on a smooth and shiny surface, the whole of light is sent back in the same direction. It is called reflection (as shown in fig). Mirrors do not allow even a small amount of light to pass through them. Mirrors show regular and complete reflection. Reflection of light When all the parallel rays reflected from a plane are not parallel, the reflection is diffused or irregular. This is not violation of law of reflection, the reason for diffused reflection is due to irregularities in the reflecting surfaces.
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| Regular reflection of light |
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| Irregular reflection of light |
Q.4. Define Lateral Inversion.
Ans. Phenomenon of changing left to right and right to left by the mirror, while forming images is called lateral inversion.
Q.5. Define the term 'Dispersion of light."
Ans. Dispersion of Light: When a beam of light passes through a prism, the white light splits into seven colours. This phenomenon is known as dispersion of light.
Q.6. Define-normal, angle of incidence and angle of reflection.
Ans. A line making an angle of 90° to the line representing the mirror at the point, where the incident ray strikes the mirror, is known as the normal. The angle between the normal and incident ray is called the angle of incidence. The angle between the normal and the reflected ray is known as the angle of reflection.
Q.7. Where we use the periscope?
Ans. Periscopes are used in submarines, tanks and also things outside by soldiers in bunkers to see.
Q.8. What do you understand by accommodation of eye?
Ans. In order to see near objects, muscles attached to the lens contract and the lens become thicker. On the other hand, muscles relax and the lens becomes thinner, when distant objects are to be seen. This changing of the thickness of the eye lens is called accommodation.
Q.9. What is Myopia?
Ans. Myopia: The focal length of the eyes of some people is very small. Therefore, the images of distant object is not formed on their retina, but it is formed at a point in front of the retina. In other words, they cannot see distant objects. This defect of vision is called 'Myopia' or short sightedness.
Q.10. What is Hypermetropia?
Ans. Hypermetropia: When people grow old, the muscles of the eyes lose their strength to control the cuvature of the lens. As a result, the image of nearby objects is not formed at the retina, but it is formed behind the retina. So, these people cannot see nearby objects clearly. This defect of vision is called Hypermetropia or Far-sightedness.
Q.11. What should be the proper diet taken by a man for healthy eye?
Ans. One should, include in the diet components, which have vitamin A. Raw carrots, broccoli and green vegetables, such as, spinach (Palak), methi, amarnth and cod liver oil are rich in vitamin A. Eggs, milk, curd, paneer, butter, ghee and fruits, such as, papaya, banana, mango, apple, dates etc. also contain plenty of vitamin A.
Q.12. You read a newspaper because of the light it reflects. Then why do you not see even a faint image of yourself in the newspaper?
Ans. We read newspaper because light is scattered by the paper into our eyes. This is called diffuse reflection. Newspaper has a rough surface. It cannot produce regular reflection. That is why image cannot be seen.
Q.13. How can we see a virtual image when it cannot obtained?
Ans. True, a virtual image cannot be taken on screen. But our eye lens forms a real image on the retina of eyes.
Q.14. Does the size of mirror affect the nature of the image?
Ans. No, size of mirror does not affect the nature of the image except that a bigger, mirror forms a brighter image.
Q.15. What would happen, if you threw the light on the mirror along the normal?
Ans. As in this case, angle of incidence is zero degree, so after reflection, light beam returns at some path, i.e., along normal
Q.16. What is the function of eyelids?
Ans. The function of eyelids is to protect eyes from any objects entering the eye. Eyelids also shut light when not required.
LONG ANSWER TYPE QUESTIONS
Q.17. What is reflection of light? State the laws of reflection.
Ans. Reflection is a phenomenon, in which a beam of lights falls on some surface and returns back in different directions. It may be regular or irregular.
Two laws of reflection are:
(a) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface lie in the same plane.
(b) The angle of incidence is equal to the angle of reflection.
Q.18. What are the characteristics of images formed by plane mirror?
Ans. Characteristics of images formed by plane mirror are :
(i) Plane mirror forms erect images.
(i) Image formed is of the same size as the object.
(iii) The distance of image from the mirror is equal to the distance of object from the mirror.
(iv) It is virtual, it can't be obtained on a screen.
(v) Image is laterally inverted.
Q.19. Differentiate between a real image and a virtual image.
Ans. Differences between a real and a virtual image.
Real Image
1. The real image is formed when the rays of light actually meet reflection.
2. Real image can be obtained on the screen.
3. Real image is always erect.
Virtual Image
1. The image formed is virtual when the rays of light don't meet after reflection.
2. Virtual image cannot be obtained on the screen.
3. Virtual image is always inverted.
Q.20. Explain the internal structure of human eye. Also discuss the functions of various parts of the eye.
Ans. Human eye is roughly spherical in shape. It has a white coloured tough outer coat. It protects the interior of the eye from any damage. Its transparent front part is called cornea. In the Cornea, there is a small opening called the pupil. The size of the pupil is controlled by the iris.
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| Human Eye |
The iris is the coloured part of the eye. The iris acts as a shutter to control the amount of light entering into the eye. Behind the pupil is a convex lens which focuses light on the back of the eye on a layer called retina. Retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.
Q.21. Write an essay on Braille System?
Ans. The most popular resource for visually challenged persons is known as Braille. This system was invented by Louis Braille, himself a blind person, in 1821 and adopted in 1932. There is Braille code for common languages, mathematics and scientific notation. Indian languages such as Hindi, Sanskrit, Marathi, Gujarati, Bengali, Tamil, Telugu and Urdu, can be read using the Braille system.
Braille system has 63 dots patterns of characters. Each character represents a letter, combination of letters, common word or
grammatical sign. Dots are arranged in cells of two vertical rows of three dots each.
These dots when embossed on Braille sheet help visually challenged to recognise words by touching. For the purpose, the dots are slightly raised.
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