h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Obviously, I know that you get a shadow by blocking light with something that's either translucent or opaque. But what exactly *is* a shadow?

(Question courtesy of a bright almost-4-year-old)

A shadow isn't really anything at all. It's just a hole in the light.

If you cut a three inch square piece out of your towel, then you have a hole. The hole isn't really anything itself. It's no different from all the air in front of the towel and behind the towel. It's only a hole because there's a towel around it.

A shadow is the same. It's a bit where the light is missing.

Those are all concepts PaperBaby understands, so I should be able to tell her that with no problems (assuming I remember by the time we get up for the day) Thanks Gnomon

Of course, I may have to remind her of the question--she hit me with it on the way home from the library yest... *glances at clock* the day before yesterday. I told her it was where the light was blocked, but I think the 'hole in the light' explanation may work better, since that's more thing-like.

If you can get a cloud of chalk dust in front of a bright light, you should be able to demonstrate that shadows are three-dimensional, rather than just shapes on the ground.

B

I like that description!

Of course shadows are not usually totally dark because of light getting round the edge of the hole.....

Unless, of course, you were blocking light from a 'laser' in an otherwise dark room then it would be totally black.

There will be some diffractioin of the light but if the solid bit is big enough the diffracted light should not meet up.

At the bottom of this link is a nice visualiastion of light diffraction.

A bit advanced for a 4year old but you can always show him how a pencil can appear bent when its put into water...and then try and explain THAT to him! it's a nice little 'trick' anyway and even if he doesn't understand will whet his appetite even more... if he asked about shadows he's got an inquiring mind anyway...a light at the end of the tunnel for the future of science.

http://www.physicsclassroom.com/Class/light/U12L1a.html

I presumed the child was a he! apologies if I have just gender reassigned your child.

Sorry, I 'assumed' not 'presumed'...wrong word totally...right, I'm going

Well, if I hadn't replaced 'Faith' with 'PaperBaby', the assumption would never have been made, so it's my fault