Question 6: Why it is not possible to see the interference where the light beams from the head lamps of a car overlap?

ANSWER

Interference of two light waves can be observed only when;
(1) The light is monochromatic; it consists of a single wavelength.
(2) The sources emitting the waves are coherent so that both the waves are also mutually coherent
(waves are said to be coherent if they have a constant phase difference and same frequency)
(3) Both waves have a constant phase relation.

These conditions are not satisfied in the case of light coming from the head lamps of the car. First the light is not monochromatic. Light received from each lamp has a random pattern of electronic movements. Therefore, the waves have random phases. So we can’t have interference patterns from the light from the head lamps of a car.