Question

Two independent monochromatic sodium lamps can not produce interference because

• A. The frequencies of the two sources are different.
• B. The phase difference between the two sources changes will respect to time.
• C. The two sources become coherent.
• D. The amplitude of two sources is different.

Answer 1

Answer: (B)

The phase difference between the two sources changes will respect to time.

Answer is A

Two sources of light are said to be coherent if they emit light which always has a constant phase difference between them. It means that the two sources must emit radiations of the same wavelength. The two independent sources cannot be coherent because of the fact that independent sources cannot maintain a constant phase difference between them. For experimental purposes, two virtual sources obtained from a single parent source can act as coherent. In such case all the random phase changes occurring in the parent source are repeated in the virtual sources also, thus maintaining a constant phase difference between them. Since the wavelength of light waves is extremely small, the two sources must be narrow and must also be close to each other.

Interference can never be obtained with two independent sources of light, such as two bulbs or two candles, due to the fact that any two independent beams of light are always incoherent.