The Brightest Part of a Shadow is in the Middle

Where is the darkest part of a shadow? I mean, the obvious answer seems to be right in the middle. If you look closely at a shadow, as you move the object away from the wall, you notice that the shadow gets a bit fuzzy. So clearly, the edges are lighter. Now, we know that light bends around corners; that's a phenomenon called "diffraction."

So, is it diffraction that's responsible for the fuzzy edges of shadows? Actually?... No. The reason that shadows have fuzzy edges is because most light sources, like the sun, are not perfectly point objects. So, light from one edge of the sun is coming at a very slightly different angle from light from the other edge of the sun, and that's what gives us these fuzzy edges. It's not diffraction.

But diffraction can play a significant role in shadows, and in fact, it was the cause of a massive debate about 200 years ago... In 1818, the French Academy sponsored a competition to try to find the best explanation of diffraction. Augustine Fresnel entered this competition with the suggestion that light is a wave phenomenon. And just like any ordinary wave, it bends as it passes around an obstacle.

But one of the judges was Simeon Poisson, a harsh critic of the wave theory. He much preferred Newton's idea that light was a stream of particles. And to show just how ridiculous the wave theory was, he showed that Fresnel's theory would predict a bright spot in the shadow, right in the middle of the shadow behind a circular object. That there would be a spot, almost as bright as if the object wasn't there at all. And he thought this was absurd.

But my question is, does it exist? Is the brightest part of a shadow really in the middle? To find out, we're gonna have to do the experiment.

So first I had to find some circular obstacles. I selected spheres instead. I got some marbles and some small spherical magnets. Then for a powerful light source and a large distance to the screen, I selected a lecture theater with a projector. Now the first thing we're gonna need is a small aperture for the light to pass through. So I'm gonna poke a hole in this card with a screw. It's important to have a small aperture because you want the light to be in phase. And that should work if it's all coming out of this small little aperture.

But when I put a sphere in front of it ... Alright, somebody hit the lights. Here we go... I can't see a bright spot. I can't really see anything. So next I tried a cellphone flashlight, putting a marble in front of that. And again, I could see nothing. I don't see anything in the middle.

The idea with Poisson's spot is that light should diffract around a circular object, or a sphere. And because the center of the shadow is equidistant from all the edges of that obstacle, all of the light should constructively interfere at that point, creating that bright spot. So next I tried an overhead projector. I tried a string of the small spherical magnets.

It's weird. Like, I feel like I can see it.

Derek: Do you think you can see a bright spot in the middle of that bottom one?

Girl: No.

Derek: No? I feel like in the very center of each one, I can see a bright spot. I don't know. My eyes are just totally going nuts up here. Like, staring at shadows and trying to see what we want. But this one is opaque. It's nice and spherical. Set it down... And if I adjust the focus... You can see there's a bright spot right in the middle of that marble.

So the spot looks pretty good, but something about it didn't sit right with me. And you know, when you think something is true, you really should try as hard as you can to disprove it. What if you used your fingers to kind of make the edges less spherical?

Yeah. You see now that, that I think is concerning, that fact we can still see a bit of spot even though it looks like you're... Right? How is the light getting through and getting there?

What was really happening was light was coming up from the base of this overhead projector, bouncing off this lens, back onto the top of the marble, and then back up through this lens, and onto the wall.

So finally I decided to use a laser. I resisted doing this earlier because in 1818 they didn't have lasers. In fact, Poisson didn't do the experiment. He didn't think that you needed to. The idea of a bright spot in the middle of a shadow was just so ridiculous. And Fresnel didn't do the experiment either.

But one of the other judges, by the name of Arago, decided to actually do the experiment. And when he did it, he saw something similar to what I saw. So here I was using the laser through a diverging lens. And then that beam was shone onto a marble resting on top of a loop of tape.

So you can see on the wall, basically exactly what you'd expect. Just a shadow of the marble. But when you turn the lights off... there it is... A bright spot right in the middle of that shadow.

So the brightest part of a shadow is, in the middle! As long as it's the shadow of a circular or spherical object. Now this spot is sometimes called Arago's spot because he did the experiment and found it. Sometimes it's called Fresnel's bright spot because it was his theory. But pretty frequently, it's referred to as Poisson's spot. A harsh reminder that it's not only your great achievements, but also your greatest mistakes that can be named after you.

We don't see Poisson's spot in our day-to-day lives for many reasons, one of which is that most objects are not perfect circles. Plus, if they have any surface roughness, really any at all, that will completely wash out Poisson's spot. And finally, most light sources are not coherent. That is, the waves are not coming all in phase: peaks with peaks and troughs with troughs.

You might think you could never see Poisson's spot under ordinary conditions. But you can... What you need to do is look at a diffuse source of bright light like a fluorescent tube or the blue sky. You should see some small light specks drifting around your visual field.

Now those are caused by floaters. Little particles actually drifting around inside your eyeball. And they can be all sorts of different shapes, but some of them are spheres. And so they cast a shadow on the back of your retina. And right in the middle of that shadow is Poisson's bright spot.

And that is what demonstrated that light really is a wave phenomenon. And you don't even have to take my word for it. You can see it with your very own eyes.