The repetition of the sound, which is reflected from a high building or any such surface, is called an echo. An echo can be heard only when the distance between the source of sound and the reflecting body is at least 17 m.

Figure below illustrates it more clearly.

For Example when a person shouts in a big empty hall he listens to his sound repeatedly.

(b) Following are the difference between echo and reverberation.

(c) The laws of reflection are the following:

1. The angle of incidence is equal to the angle of reflection.

2. The incident ray, the normal at the point of incidence and the reflected ray - all lie in the same plane.

Activity:

Materials required: a drawing board, a white sheet of paper, a few common pins, a protractor, pencil, eraser and a plane mirror.

Method:

1. Fix the white sheet of paper firmly on the drawing board. Place the plane mirror on it and trace its outline on the paper.

2. Remove the mirror and draw the normal.

3. Now place the mirror again on the outline. The normal will be reflected clearly on the mirror.

4. Next place two pins in a straight line on one side of the normal on the white sheet of paper.

5. Next place two pins on the other side of the normal in such a way that these two pins is in a straight line with the reflection of the two pins on the other side of the normal.

6. Now remove the mirror and the pins and join the pin marks to the normal.

7. Measure the angle of incidence and the angle of reflection. Both will be equal, proving the first law of reflection.

Result: Since the lines representing the normal, the incident ray and the reflected ray are all represented on the same sheet of paper, the second law is also verified.