NCERT Class 8 Science Light

LIIGHT

The world is largely known through the senses. The sense of sight is one of the most important senses. Through it we see mountains, rivers, trees, plants, chairs, people and so many other things around us. We also see clouds, rainbows and birds flying in the sky. At night we see the moon and the stars. You are able to see the words and sentences printed on this page. How is seeing made possible?

16.1 What makes Things Visible

Have you ever thought how we see the various objects? You may say that eyes see the objects. But, can you see an object in the dark? It means that eyes alone cannot see any object. It is only when light from an object enters ou  eyes that we see the object. The lightmay have been emitted by the object, or may have been reflected by it. You learnt in Class VII that a polishedor a shiny surface can act as a mirror. A mirror changes the direction of light that falls on it. Can you tell in which direction the light falling on a surface will be reflected? Let us find out.

16.2 Laws of Reflection

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. Draw lines showing the position of the plane mirror, the incident ray and the reflected ray on the paper with the help of your friends. Remove the mirror and the comb. Draw a line making an angle of 90º to the line representing the mirror at the point where the incident ray strikes the mirror. This line is known as the normal to the reflecting surface at that point (Fig. 16.2). The angle Do you see any relation between the angle of incidence and the angle of reflection. Are they approximately equal? If the experiment is carried out carefully, it is seen that the angle of incidence is always equal to the angle of reflection. This is known as the law of reflection. Let us perform another activity on reflection.

Bhoojo remembered that he had studied in Class VII some features of the image of an object formed by a plane mirror. Paheli asked him to recall those features:

(i) Was the image erect or upside down?

(ii) Was it of the same size as the object?

(iii) Did the image appear at the same distance behind the mirror as the object was in front of it?

(iv) Could it be obtained on a screen? Let us understand a little more about the formation of image by a plane mirror in the following way:

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. What does it indicate? It 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. Paheli and Bhoojo performed the above activities outside the classroom.

Exercises

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

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

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

4. State the laws of reflection.

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.

6. Fill in the blanks in the following :

(a) A person 1 m in front of a plane mirror seems to be _______________ m 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..

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

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.

9. Describe the construction of a kaleidoscope.

10. Draw a labeled sketch of the human eye.

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 teachers advise?

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

13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Please refer to attached file for NCERT Class 8 Science Light