Question 7: A telephone pole casts a clear shadow in the light from a distant head lamp of a car, but no such effect is noticed for the sound from the car horn. Why?

ANSWER

Both light and sound are waves and therefore, follow the same laws. Still sound wave from the horn of a car can be heard behind a telephone pole (equally as somewhere else at the same distance) but the light from its head light cast shadow. This is due to the diffraction phenomena of waves.

Diffraction is the phenomena of bending of light and other waves round the corners of obstacles.

However, it follows the rule that the wavelength of the incident wave should be of the order (that is either greater than or equal to) of the width of the opening or obstacle. Now the wavelength of light is ranging from 400 to 700 nano meters which is very much less than the dimensions (length, breadth) of the pole. On the other hand, the wavelength of the sound waves ranging from 17 mm to 17 m which is greater than the width of the pole.Therefore, diffraction can be observed in the case of sound and not in the case of light. So the sound waves bend round the pole and the light waves can not.Therefore, the sound of the horn from the car can be heard behind the pole and light from its head lights will cast shadow behind the pole.