Solution to Activity 11.5 of NCERT Class 9 Science Chapter 11 Work and Energy

5. Complete Activity 11.5 (Page 133).

• Take two identical pipes, as shown in Fig. 11.9. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown.

• Arrange them on a table near a wall.

• Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.

• Adjust the position of the pipes so that you can best hear the sound of the clock.

• Now, measure the angles of incidence and reflection and see the relationship between the angles.

• Lift the pipe on the right vertically to a small height and observe what happens. (In place of a clock, a mobile phone on vibrating mode may also be used.)

Answer:

Aim: To find the relationship between the angles of incidence and reflection and also to observe what happens when the pipe on the right is lifted and conclude based on that.

Materials Required: Chart paper, table, clock.

Procedure:

(i) Make two identical pipes using chart paper, making sure that the length of the pipes is sufficiently long.

(ii) Place them on a table against a wall.

(iii) Now, place the clock in front of the opening of the pipe on the left and try to hear the sound of the clock through the pipe on the right.

(iv) Adjust the position of the pipes until you can best hear the sound of the clock.

(v) Now measure the angles of incidence and reflection with respect to the normal and note the relationship between the angles.

(vi) Now lift the pipe on the right to a certain height and observe what happens.

Observations:

The angle of incidence and the angle of reflection with respect to the normal are equal.
When the pipe on the right is lifted to a certain height, the sound cannot be heard anymore.

**Reflection of Sound:** The angle of incidence and angle of reflection will be equal. What happens when you lift the pipe on the right?

Conclusions:

We conclude that

In case of reflection of sound, the direction of incident ray and the direction of the reflected ray make equal angles with the normal.
The incident ray, reflected ray and normal all lie on the same plane. Hence, when we lifted the pipe on the right vertically upwards we could not hear the sound.

“Take two identical pipes, as shown in Fig. 11.9. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown.

Arrange them on a table near a wall.

Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.

Adjust the position of the pipes so that you can best hear the sound of the clock.

Now, measure the angles of incidence and reflection and see the relationship between the angles.

Lift the pipe on the right vertically to a small height and observe what happens. (In place of a clock, a mobile phone on vibrating mode may also be used.)” – Solved.