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NCERT Solutions for class 8 science Chapter 15 Light

class 8th science Chapter 15

NCERT Solutions Science for Class 8 Chapter 15 Light

class eight science Chapter 15 question answer

Exercises
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.
Answer :- You cannot see the objects kept in that room by staying in a dark room, but you can see the objects outside the room, because when the light hits an object and gets reflected and falls on the retina of our eyes, then we see the objects.

2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer :- The difference between regular and diffuse reflection is as follows –

Regular Reflection	Diffuse Reflection
When light falls on a smooth, polished and regular surface, it is called regular reflection.	When all the parallel rays are not parallel after reflecting from a rough or irregular surface, then such reflection is called diffuse reflection.
Example: mirror or polished wood	Subject reflective page from cardboard piece of paper

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) Polished wooden table	(b) Chalk powder
	(c) Cardboard surface	(d) Marble floor with water spread over it
	(e) Mirror	(f) Piece of paper

Answer :- 
(a) Polished wooden table – There will be regular reflection, because this smooth and shiny surface.
(b) Chalk powder – There will be diffuse reflection, because it is not shiny.
(c) Cardboard surface – There will be diffuse reflection because it is rough and without glare.
(d) Marble floor with water spread over it – There will be regular reflection, because it is smooth and shiny.
(e) Mirror – Has regular reflection, smooth and shiny surface.
(f) Piece of paper – Diffuse reflection because of rough and unglazed surfaces.

4. State the laws of reflection.
Answer :- There are two laws of reflection –
(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the reflected ray and the normal drawn at the point of incidence on the reflecting surface lie in the same plane.

5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Answer :- To show that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. Will use the following –
Activity – Use a sheet of hard paper or chart paper. The sheet should be slightly protruding from the edge of the table (picture). Cut the protruding part of the sheet from the middle. Look at the reflected ray. Make sure that the reflected ray is also visible on the protruding part of the paper. Fold the protruding part of the paper where the reflected ray is visible. Can you still see the reflected ray? Return the paper to its initial state.

6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image.
(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.
Answer :- 
(a) A person 1 m in front of a plane mirror seems to be 2m m away from his image.
(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with right.
(c) The size of the pupil becomes larger when you see in dim light.
(d) Night birds have less cones than rods in their eyes.

Choose the correct option in Questions 7 – 8
7. Angle of incidence is equal to the angle of reflection.

(a) Always	(b) Sometimes
(c) Under special conditions	(d) Never

Answer :- (a) Always

8. 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.
Answer :- (a) virtual, behind the mirror and enlarged.

9. Describe the construction of a kaleidoscope.
Answer :- To make a kaleidoscope, take three rectangular strips of mirror about 15 cm long and 4 cm wide. Connect them in the shape of a prism as shown in the picture. Fix them firmly in a cylindrical tube made of cardboard or thick chart paper. Make sure the tube is slightly longer than the mirror strips. Close one end of the tube with a cardboard disc with a hole cut out to allow a view of the inside. To make the disc durable, stick a sheet of transparent plastic below it. At the other end of the tube, fix a circular plate of flat glass firmly touching the mirrors. Place some small pieces of colored glass (pieces of colored bangles) on this plate. Close this end of the tube with a worn glass plate. Leave enough room for movement of the colored pieces.

10. Draw a labelled sketch of the human eye.
Answer :- The shape of our eyes is almost circular. The outer covering of the eye is white. It is hard so that it can protect the internal parts of the eye from accidents. Its outer part is called cornea or clear circle. Behind the cornea we find a dark colored muscular structure called the iris. There is a small opening in the iris which is called the pupil. The size of the pupil is controlled by the iris.
The iris is the part of the eye that provides its distinctive color. Behind the pupil is a lens that is thicker at the center.

11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Answer :- Gurmeet wants to do the activity using laser torch, but the teacher forbids him to do so because the light of laser torch is very bright and it can harm the eyes.

12. Explain how you can take care of your eyes.
Answer :- To keep your eyes safe, you can take care of your eyes in the following ways –
(i) Get your eyes checked regularly.
(ii) Too bright light is harmful for the eyes, avoid it. Do not look at sunlight directly.
(iii) Do not read/work in low light.
(iv) Do not rub your eyes.
(v) Keep reading material, mobile, computer at a sufficient distance.

13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer :- If the reflected ray makes an angle of 90° with the incident ray, then the value of the angle of incidence will be 45°.

14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer :- If two parallel plane mirrors are placed at a distance of 40 cm from each other, then infinite images of a candle placed between them will be formed.

15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.
Answer :-

16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer :- Boojho will not be able to see himself. He can see the images of objects situated at points P and Q but cannot see the image of the object situated at point R.

17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?

(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
Answer – The image of an object placed at A will be formed at the same distance behind the mirror as the object is placed in front of the mirror.

(b) Can Paheli at B see this image?
Answer :- Yes

(c) Can Boojho at C see this image?
Answer :- Yes

(d) When Paheli moves from B to C, where does the image of A move?
Answer :- Will not move