You've probably never spent too much time thinking about your shadow, but if you're outside or under bright lights right now, take a moment to look at it. It's okay, we'll wait. Now consider where the darkest part is. At first glance, it seems like it must be the centre, because as you start to move your hand or body away from your shadow, it gets lighter and fuzzier at the edges. Case closed, right? Well, as Derek explains in the latest episode of Veritasium, it's not that simple.
What makes things more complicated is that light behaves like a wave, which means it can bend around corners and objects - a phenomenon known as diffraction. When this was first proposed in 1818, it was ridiculed by some scientists, and the main argument was that if light does bend around objects, it should bend around perfectly round objects in a way that causes a bright spot right to appear the middle of their shadows. Ridiculous, right?
As we now know, the wave-like nature of light was eventually confirmed. But what about the bright spot in the middle? Does that really happen?
To put the hypothesis to the test, Derek sets up a whole range of experiments, all of which you can try at home with the right equipment. And after much trial and error, he manages to show that a bright spot does appear.
Here's why: when you shine bright light from a point light source at a perfectly round object, the waves of light are all travelling 'in phase', which means they're moving in sync. That also means they'll bend around a perfectly round object in exactly the same way, before refracting and meeting up in the centre of the shadow.
The reason we don't see this bright spot in our normal shadows is that we're not perfectly round - sorry - and the Sun also isn't a totally even point light source.
But there IS a way you can see this happening with your own eyes - and without all the lasers and perfect spheres. We'll let Derek explain that, but it has a lot to do with those floaters you see swimming around in your eyes sometimes. Watch the video, and never think of your shadow in the same way again.