NCERT Solutions for Class 8 Science Chapter 16 Light

1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.

Answer: In dark room objects are not visible. Objects outside the room are visible. We can view object only when light from the object reflects and reaches to our eye. In dark room, due to absence of light, no light is reflected thus, objects are not visible. Outside the room in presence of light, light rays reflects, and thus objects are visible.

2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:

Regular Reflection	Diffused reflection
Occurs on smooth or regular surface	Occurs on irregular surface
Reflected rays are parallel to each other to parallel incident rays	Reflected rays are not parallel to each other to all parallel incident rays
E.g. Reflection by a plane mirror	E.g. Reflection by ground surface

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 – A polished surface shows regular reflection. Polished wooden table is an example of smooth surface, thus, it shows regular reflection.

(b) Chalk powder – A chalk powder is an example of irregular reflection. As the surface will not be smooth, therefore, diffused reflection will occur.

(c) Cardboard surface – Cardboard surface is an example of diffused reflection. Surface of cardboard is not smooth thus, it shows diffused reflection.

(d) Marble floor with water spread over it – It shows regular reflection. This is because marble floor spread with water is a smooth surface.

(e) Mirror – It shows regular reflection. This is because mirror surface is a smooth surface.

(f) Piece of paper – Piece of paper will have a diffused reflection. This is because, it have a rough surface.

4. State the laws of reflection.

Answer: Laws of reflection are as follows –

1. Angle of reflection is always equal to angle of incidence.
2. Incident ray, reflected ray and normal to the reflecting surface all lie in 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:

Take a mirror and arrange on table as shown in the figure. Take a sheet of paper. Now make a hole in its centre. Hold this sheet of paper as a normal to the table. Place another sheet of paper vertical to the mirror as shown in mirror. On this sheet, draw a normal from the mirror. Light a torch, such that light rays fall on the mirror through the small hole in the sheet of paper. These rays of light should fall on normal at the bottom of the mirror. When light falls on the mirror it gets reflected. Incident ray, normal and reflected ray can be easily observed on the sheet of paper placed on the table. Thus, it suggest that all three lie in the same plane.

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.

Answer: 2

(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 ____________.

Answer: left

(c) The size of the pupil becomes ____________ when you see in dim light.

Answer: large

(d) Night birds have ____________ cones than rods in their eyes.

Answer: less

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. Law of reflection states that, angle of incidence is always equal to angle of reflection.

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: (b) virtual, behind the mirror and of the same size as the object

9. Describe the construction of a kaleidoscope.

Answer: Construction of Kaleidoscope –

Take three rectangular strips of length 15 cm and breadth 4 cm. Join three strips together to form a prism. Fix this prism in slightly long circular cardboard tube. Close the circular tube with cardboard disc containing hole. Fix a plane glass plate on the other end of the tube. Make sure that glass touches the prism. Place small broken pieces of colored glass on this glass plate. Close this end by round glass plate such that there is enough space for colored glass to move.

Click To Watch video – Construction of Kaleidoscope

10. Draw a labelled sketch of the human eye.

Answer:

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: Laser light is of high intensity. This can cause damage to retina and lead to blindness. Therefore it is advised not to use laser torch.

12. Explain how you can take care of your eyes.

Answer: Following are some ways to take care of eyes –

1. Do not read in dim light
2. If dust particles or insects enter in the eye wash your eyes quickly
3. Do not see directly to the sun
4. Keep distance of 25 cm between eyes and book while reading
5. Consult to doctor in case of an emergency
6. Do regular eye checkup

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: Law of reflection says that angle of incidence is same as angle of reflection. Thus angle of reflection and angle of incidence both are 90/ 2 = 45°. Therefore, angle of incidence is 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 mirrors are placed parallel to each other then infinite number of images 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: By using first law of reflection, path of reflected light can be obtained. From the given diagram it can be observed that, light reflects from second mirror from an angle of 60°.

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: Image formed by plane mirror is virtual image formed behind the mirror. Image formed is far behind as object is placed in front of it. A cannot see his image. This is because length of mirror is short on his side. But he can see object placed at point P and Q. Also, he cannot see object at point R.

17. (a) Find out the position of the image of an object situated at A in the plane mirror.

(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?

Answer: Image of object placed at A formed behind the mirror. For A image distance is equal to the object distance. Thus, image of A can be shown as follows. Yes, Paheli at B can see the image. Boojho at C can see the image. Image will remain at same place when paheli and boojho are move from B to C.

Check other lessons NCERT Solutions for Class 8

Download free NCERT textbook class 8 Science – Chapter 15 Some Natural Phenomena