Thread: Shadow

When you distance your finger from the wall in the direction of a light source, the shadow becomes less and less dark? The way light behaves, I would expect my finger to block more light making the shadow bigger and just as dark.

How does the light make it's way around your finger on the wall if radiation travels in a straight line?

I'd be inclined to say that it is due to the scattering of light- so when your finger is further away from the wall, more scattering takes place behind your finger allowing the light to reach the wall in higher intensities. I may be wrong, though.

A lot has to do with the light source. You are unlikely to have access to point source and are using something like a light bulb. Since a light bulb is much wider than your finger, it can't block all of the light from the bulb and light "leaks' around the edges to fill in the shadow. The further from the wall and closer to the light your finger is, the greater this effect.


For example, here are two shadows for the same object, one made by a point light and one made by a larger source.




Notice how the shadow for the one is dark in the center then quickly fades away. In a real situation you would also have reflected light coming in from other directions which would obscure the lighter parts of the shadow, so you might only notice the smaller dark spot.

Makes sense. I've seen the shadow of an air plain flying high overhead shaped exactly like the plain. If you were to put the plain near the sun, the plain would only block a tiny percentage of photons radiating from the sun, so I guess that means the rest of the photons would just about completely obscure the shadow.

Here's the same effect on a really large scale; http://en.wikipedia.org/wiki/Eclipse
Pay particular attention to the terms "umbra" and "penumbra".

You know, this changes the way I think about an object that emits radiation. I've always pictured a sphere shooting photon perpendicular evenly around the sphere. But in fact, each point on the sphere is shooting light in every direction, even inward.