> Speed "why are there shadows?"
> Mom "because you block the light"
> Speed "a more interesting answer?"
At that point she punted to me. Science to the rescue!
Ah, but shadows aren't always because you're blocking light from a reflecting surface, although that's the simplest way of making them!
You could also be refracting the light away with a lens, for example, because light gets bent through an angle anytime it goes from something less dense (like air) to more dense (like glass or water), or the other way around. When you make a bright spot with a magnifying glass there's always a shadow all around it - try it and see. That's also why you see moving shadows on the bottom of swimming pools, or over hot surfaces where streams of hot air are mixing with cooler air (hot air is less dense than cool air). The light's being deflected away from one spot and piling up in another, making shadows and bright spots - anti-shadows!
If you shine a light past something really, really massive like a planet or a black hole, the gravity can also bend the light, so as well as making a shadow exactly behind the massive object (by blocking the light), you're also making a different kind of shadow all around it by bending the light's path - and an anti-shadow directly behind it! Like this:
Black hole lensing (Picture courtesy of the Wikipedia entry on gravitational lensing.)Also, because light acts as a wave, you can even make shadows by putting two light beams together out of phase! To do this in a way that you can see you need to use light of exactly the same color and with all the light traveling in the same direction, which is called coherent light - that's the kind of light that comes out of a laser. If the light wave from one laser beam is getting highest just as the other one is getting lowest, you get a shadow where they cancel out, even though nothing is blocking the light!
The easiest way to get two beams of coherent light is to split the beam from one laser using slits in a screen. Here's a simulation.
So there are lots of interesting ways shadows can be made, besides blocking the light!
2 comments:
You can also make shadows in one band of light but not another. For example, take a piece of metal that is transparent to the UV part of the spectrum and shine sunlight on it. The visible light is blocked and makes a shadow. The UV light passes right through and leads to no shadow. Of course, we do not usually have UV sensors handy so demonstrating this might be difficult.
Good point! Or, to be more visible, you could use a piece of colored glass or plastic. Funny how we call a shadow cast by a green bottle a "green shadow" when that's really the only color that isn't casting a shadow!
Post a Comment