Poisson's Spot

This is a part of the Eureka! Stories web site. These are stories for children about famous historical scientists. They are under constant development, so check back for additions.

Waves and Particles

For years and years scientists tried to find out what light was. One of the biggest arguments was whether it acted like particles - that is like tiny balls, or whether it acted like waves - like the ripples on water when you drop something into it.

We now know that very small things don't act exactly like either waves or particles they act a little bit like both. Light is made up of very small particles called photons which each act like they are a part of a wave - even if there's no other photons around.

Fresnel and Poisson

In 1818 the French Academy held a competition for scientists. Fresnel entered with a paper about diffraction of light. Diffraction is when a wave bends to get behind something. Particles don't do that. If you hold a shield in front of you, you don't have to worry about things bending around it to get at you. However, waves do diffract. If you watch a wave go past something like a big rock in the water, you see that it bends and gets in behind the rock. If light diffracts then it is a wave. One of the judges, Poisson, hated the wave theory. He was sure that light was made of particles.

Fresnel's paper was brilliant, but Poisson was determined that it couldn't win. So Poisson thought about it and said that it light diffracts then shadows can't be completely black, some of the light bends to get behind. If you look at the shadow of a small circular object, the light bending from all sides would add up especially strongly in the middle. This means that in the middle of the shadow, there would be a faint bright spot.

Poisson was now convinced that he had shown how ridiculous the wave theory really was. He presented his idea to the other judges to prove that they couldn't possible make Fresnel the winner.

Another judge, Arago, decided to test it for himself. He did the experiment as Poisson had described and discovered - to everybody's surprise - that there was indeed a bright spot in the middle of the shadow! So instead of proving that Fresnel was wrong, Poisson had done something very important for proving that Fresnel was right - he had used Fresnel's theory to make a prediction that was so wild sounding that if it was true it almost certainly meant that Fresnel's theory was right.

Poisson had to give up! He had proven the thing he wanted to show was wrong. Fresnel won the contest, and the spot was named Poisson's Spot. This gives him the credit he deserves for his part of proving the theory to be true, but it was also very embarrassing for him.