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Interference

Interference patterns show that light is a wave.

The particle theory of light could not explain interference patterns. Only the wave theory could.

All waves can create interference patterns. When similar waves from different places overlap, they can either reinforce each other to create a bigger wave or cancel each other out to produce no wave at all.

Thomas Young discovered interference patterns in 1803.

We now know that supernumerary rainbows, seen in the sky for millennia, are an interference pattern.

When Young made his discovery, explaining double refraction was still a problem for the wave theory. Young eventually realised that a transverse wave—one that vibrates at right angles to the direction it travels—could explain double refraction.

Despite this evidence for the wave theory, many scientists remained unconvinced. It was known by this time that light was absorbed in chemical reactions at a single spot, which suggested that light was a particle. Moreover, despite his discoveries, Young could not give a complete account of how interference worked.

However Augustin Fresnel did develop a mathematical theory of interference. This theory suggested that in the middle of a circular shadow there should be a bright spot of light. Although it was thought ridiculous, this idea was tested—and proved correct.

This dramatic test convinced almost everyone that light was a wave.


Interference in surface waves
magnifyInterference in surface waves


Young
magnifyThomas Young


Young's interference diagram
magnifyYoung's interference diagram


Supernumerary rainbow
magnifySupernumerary rainbow
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