MP Board Class 8th Science Solutions Chapter 16 Light
MP Board Class 8th Science Light NCERT Textbook Exercises
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
We cannot see objects in the dark room. But we can see objects outside the room, in case there is light outside the room. To see an object, we need light which must reflect from the object to the eyes.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Following are the differences between a regular reflection and diffused reflection:
No, diffused reflection is not due to failure of laws of reflection.
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) Polished wooden table
(d) Chalk powder
(b) Cardboard surface
(e) Marble floor with water spread over it
(f) Piece of paper.
(a) A polished wooden table will have regular reflection because its surface is smooth.
(b) A chalk powder will get the diffused reflection because chalk particles are not smooth.
(c) Cardboard surface has minute irregularities on it, so it will have a diffused reflection.
(d) Marble stone with water spread over it will have regular reflection if water is stable but if waves are formed then diffused reflection will take place.
(e) A mirror will reflect light regularly due to its smooth surface.
(f) A piece of paper will have irregular reflection due to minute irregularities on it.
State laws of reflections.
There are two laws of reflection:
- The angle of incidence is always equal to the angle of reflection. Thus, Zi = Zr
- The incident ray, the reflected ray and normal to the mirror at the point of incidence, all lie in the same plane.
Describe an activity to show the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Fix a white sheet of paper on a drawing board or a table. Take a comb and close all its openings except one in the middle. You can use a strip of black paper for this purpose. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side (Fig. 16.3) With slight adjustment of the torch and the comb you will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of the light ray (Fig. 16.3).
After striking the mirror, the ray of light is reflected in another direction. The light ray, which strikes any surface, is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray. Perform above activity again. This time use a sheet of stiff paper or a chart paper. Let the sheet project a little beyond the edge of the table (Fig 16.4). Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls.
Fig. 16.4: Incident ray, reflected ray and the normal at the point of incidence lie in the same plane
When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When you bend the paper you create a plane different from the plane in which the incident ray and the normal lie. Then you do not see the reflected ray. This indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is another law of reflection.
Fill in the blanks in the following:
(a) A person 1 m in front of a plane
(b) If you touch your ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with
(c) The size of the pupil becomes when you see in dim light.
(d) Night birds have cones than rods in their eyes.
(b) left, left hand
Choose the correct option in Questions 7 – 8:
Angle of incidence is equal to the angle of reflection
(c) Under special conditions
Image formed by a plane mirror is:
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
(b) virtual, behind the mirror and of the same size as the object.
Describe the construction of a kaleidoscope.
Kaleidoscope is an optical instrument used to see a number of beautiful patterns. It is made up of a circular cardboard tube or tube of a thick chart paper in which rectangular mirror strips are joined together to form a prism. At one end of the tube, touching these mirrors, a circular plane glass plate is fixed and several small pieces of coloured glass (broken pieces of coloured bangles) are placed upon it. This end is closed by ground glass plate. Allow enough space for the colour pieces to move around. Now the kaleidoscope is ready. When we peep through the hole, we will be able to see a variety of patterns in the tube.
Draw a labeled sketch of the human eye.
Gurmit wanted to perform Activity 16.8 (of textbook) using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?
The Teacher advised her so because laser light is harmful for eyes and can cause some permanent damage to her eyes when throw directly in eyes.
Explain how you can take care of your eyes?
It is necessary that we take proper care of our eyes. If there is any problem we should go to an eye specialist. Have a regular checkup. We must:
- If advised, use suitable spectacles.
- Too little or too much light is bad for the eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, or powerful lamps, can injure retina, which is very delicate.
- Never rub the eyes if any small particle or dust goes into the eyes. Wash the eyes with clean water. If condition does not improve, go to a doctor.
- Wash the eyes frequently with clean water.
- Always read at the normal distance for vision.
- Do not look at the sun or a powerful light directly.
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
We know that,
Thus, the angle of incidence is 45°.
How many images of a candle i will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Infinite number of images of the candle will be formed, if it is placed between two parallel plane mirrors.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.6. Draw the reflected ray from the second mirror.
Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.8. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?
Boojho will not be able to see himself in the mirror. But he can see image of objects situated at P and Q.
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.9).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C
(a) Fig. 16.7 (a)
(id) No change as position of A is not changing
MP Board Class 8th Science Light NCERT Extended Learning – Activities and Projects
Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In vehicle case you see yourself better and why?
Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes.
Meet an eye specialist. Get your eye sight checked and discuss how to take care of your eyes.
Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children.
MP Board Class 8th Science Light NCERT Additional Important Questions
A. Short Answer Type Questions
What do you understand by the refraction of light?
The deviation (bending) of a ray of light when it passes from one transparent Medium to another, is called refraction of light.
What happens when light passes from one transparent medium to another transparent medium?
The velocity of light changes and also the direction of movement of light changes.
What is refracted ray?
The ray of light emerging in the
B. Long Answer Type Questions
Where should an object be placed for using a convex lens as magnifying glass?
Object should be placed between focus and lens.
How does retina send sensations to brain?
Retina sends sensations to brain through nerve cells.
What is a mirror? What kind of image is formed in a mirror?
Any smooth and shining surface can act as a mirror. A mirror is a smooth surface which is polished from its back and can form the image of the object which is placed in front of it. The image formed by a mirror is a virtual and erect and is of the same size as that of the object in front of it.
What is dispersion of light?
A beam of white light consists of seven colours. When a beam of white light passes through a prism, it splits into its seven constituent colours. The splitting of white light into its seven constituent colours is called dispersion of light. Rainbow is a natural phenomenon showing dispersion.
Write a short note on human eye.
Our eyes are shaped like a ball. The eye lens in the front of the eye ball makes a real and inverted image of the objects on the retina, which is located behind it. The retina is a thin tissue that has many layers of cells. The most important layer of the retina consists of nerve cells that are sensitive to the brightness as well as to the colours of light. The nerve cells carry the message about the image from the retina to the brain in the form of special signals. The brain then interprets these signals and enables us to see the object we are looking at.