Why don’t we have interference when two candles are placed close to each other and the intensity is seen at a distant screen? What happens if the candles are replaced by laser sources?
For interference effects to be prominent we need monochromatic, coherent sources of light. By this we mean that the incoming light wave should be of the same wavelength and should be in phase with each other. Two candles are not coherent neither completely monochromatic. Hence, we won’t get a desired interference pattern.
Laser sources fulfil the criteria of being monochromatic and coherent and hence will show interference.