The question of phase

We might expect from past experience that Young would have used two lamps in his demonstration of interference, in the same way that two oscillators may be used to demonstrate the interference of mechanical waves. He would not have been successful because of the way light is produced.

The basic condition for interference between two waves of the same frequency is that the phase difference between them remains constant. The phase difference between sthe light waves from two lamps varies in a completely arbitrary way.

The light from a lamp consists of an enormous number of individual wave trains which have no fixed phase relationship between them (like crowds streaming into a soccer match) and the phase varies in a completely random way. We cannot use the light from two lamps to demonstrate interference. In interference experiments such as Young's slits, the two wave trains originate from the same source and are therefore coherent. Any random phase changes occur simultaneously in the light from both slits.

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