Question

Why do we not have interference when two candles are placed closer to each other and the intensity is seen at a distant screen? What happens if the candles are replaced by laser sources?

Answer 1

Candles are the non coherent source and these sources do not have the same frequency and difference between phases is constant while interference patterns occur with the coherent sources only with using the particles medium or waves medium. Hence, two candles are not coherent source causes of the not occurring interference pattern on screen only we can see intensity at a distant screen.

Complete answer:
Coherent sources have the same wavelength, same frequency and constant phase otherwise we can say they have the same amplitude at the same distance in a wave. Two sources are said to be coherent sources when the waves emitted from them have the same frequency and zero or constant phase difference. Such as, Example: Laser; laser is a device which has a coherent source they use the double slit process to form the interference pattern.
Interference pattern formed when any sources make the crest and trough surround point of influence, hence double crest and double trough intersect to each other and formed the destructive interference and constructive interference which proffered a phenomenon of interference pattern.
This phenomenon of interference can be experimented by using a double slit method of any same frequency emitting device and this is proved by young’s double slit.

Note:
There are some properties to make the interference pattern, Amplitudes of two waves should be equal and two coherent sources should be narrow, the sources of light should be strong, two sources should be very close to each other and the source should be monochromatic light.