Science Class 8 – Chapter 16- Light – NCERT exercise solution (Question-Answer) is provided below. Total 17 Questions are in this NCERT Exercise, all are solved here.
Q1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer– No, we can’t see the object in a dark room because of the absence of light rays, which enter in our eyes after reflection which makes us able to see the objects.
On the other hand, we can see the objects outside the room because there is a lot of rays of light which comes in our eyes after reflection from the object which makes us able to see the object.
Q2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the law of reflection?
It takes place from the smooth or regular surface.
Rays are parallel.
Reflected rays are in one direction.
It takes from a rough or irregular surface.
Reflected rays are scattered in different directions.
No, diffuse reflection doesn’t mean the failure of the laws of reflection.
Q3. 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
Answer- Regular reflection because the surface is plane and polished.
(b) Chalk powder
Answer- Diffused reflection because the surface is rough.
(c) Cardboard surface
Answer- Diffused reflection because the surface is rough.
(d) Marble floor with water spread over it
Answer- Regular reflection because the surface is smooth and plane.
Answer- Regular reflection because of the surface plane and polished.
(f) Piece of paper
Answer – Diffused reflection because the surface is rough.
Q4. State the laws of reflection.
Answer- There are two laws of reflection-
The angle of incidence is equal to the angle of reflection.
Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie on the same plane.
Q5. 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.
Activity– To show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Material Required: plane mirror, torchlight, paper etc.
Procedure:- Fig 1 from NCERT book, page no, 201
Fix a white sheet of stiff paper on a table. Let the sheet project a little beyond the edge of the table. Take a comb and close all its opening excepts one in the middle with the help of black paper. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side so that ray of light can be seen along with the paper on the other side of the comb. Place a mirror in the path of the light ray. Ray of light strikes the mirror called incident ray. After striking the ray of light reflected in another direction called reflected ray.
Fig 2 from NCERT page 201
Now cut the projecting portion of the sheet in middle. We observe that reflected ray extends to the projected portion of the paper. Bend the part on which reflected ray falls. The reflected ray fall along this part can no longer be seen.
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 the right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ________.
Answer– left, left hand
(c) The size of the pupil becomes _____________when you see in dim light.
(d) Night birds have _____________cones than rods in their eyes.
Q7. The angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions (d) Never
Answer– (a) Always
Q8. The 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- (b) virtual, behind the mirror and of the same size as the object.
Q9. Describe the construction of a Kaleidoscope.
Answer– Kaleidoscope is a device based on the principle of multiple reflections.
Construction of Kaleidoscope-
It consists of three rectangular mirror strips each about 15 cm long and 4 cm wide which is inclined at an angle of 60 degrees one another forming prism. Fix this arrangement of the mirror in a tube of thick chart paper having a hole at its centre. To the other end touching the mirror plane glass plate is fixed on which broken pieces of coloured bangles are placed. This end of the tube is closed by a ground glass plate.
Now, our Kaleidoscope is ready.
Q10. Draw a labelled sketch of the human eye.
Q11. 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 advice?
Answer– Gurmit’s teacher advised her not to do because laser light is very harmful to the eyes. It can cause a permanent defect in the eye or even lose eyesight when laser torch is directed over the eyes.
Q12. Explain how you can take care of your eyes.
Answer– Our eyes are the most important and precious part of our body. We must take proper care such as-
Don’t look directly at the sun.
Always read/ write in the proper light
Wash with clean water at least twice a day.
Maintain distance between book and eye while reading at least 25 cm.
Q13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer– Here given that, reflected ray is at an angle of 90° to the incident ray i.e ∠ i + ∠ r = 90° ……(1)
To find – Angle of incidence of the ray i.e ∠ i
We know, ∠ i = ∠ r ( according to the first law of reflection of light)….(2)
Putting equation (2) in equation (1)
∠ i + ∠ i= 90°
2∠ i = 90°
∠ i = 45°
So, angle of incidence = ∠ i = 45°
Q14. How many images of a candle will be formed if it is placed between two parrel plane mirrors separated by 40 cm?
Answer– When two mirrors are placed parallel to each other and a candle is placed between them, then the infinite number of image of the candle will be formed.
Q15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30 degrees as shown in Fig. 16.19. Draw the reflected ray from the second mirror.
Answer– The laws of reflection are used to obtain the path of reflection of light.
It can be observed that the given ray of light will reflect from the second mirror at an angle of 60 degrees.
Q16. 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– No, Bhoojho can’t see himself in the mirror. He can see the image of the object at P, Q but not R.
(a) Find out the positions of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
Answer- Yes, Paheli can see this image of A.
(c) Can Boojho at C see this image?
Answer- Yes, Boojho can see the image of A.
(d) When Paheli moves from B to C, where does this image of A move?
Answer- Image will not move as the object is not moving.