I feel nerdier than usual today, so here goes...

A math teacher at my school told me about an assignment his six-year-old son had got at his pre-school.

(You need to know two things. First, this math teacher has been hired to teach students with learning disabilities, so he is very good at explaining things to kids who aren't bright, but he isn't qualified to teach "normal" students in the 16-19 year bracket. Second, in Sweden kids start first grade when they are seven, but pre-school at age six is pretty nearly compulsory.)

Anyway. The assignment the six-year-old had received from his pre-school was to discuss with his parents where the light from a lamp goes when you have turned the lamp off. Imagine it's dark outside and you are in a room with just one lamp. Turn it off and the room will be dark. Where did the light go? (Tough assignment for a six-year-old, or what do you think?)

When this teacher told me about his son's assignment, I needed to try to figure out for myself where the light goes when you turn off the lamp. If there is a window in the room, it is easy to imagine that the light escapes through that window at the speed of light, whereupon it will eventually spread in all directions until it escapes into the universe. But what if there are no windows? No chinks or holes in the walls or the doorjamb? No keyhole? No opening at all through which the light can escape? Where, then, does the light go?

Okay. Let's start by imagining that the walls, roof and ceiling of the room are painted very dark. Dark objects absorb much of the light that reaches them and reflects very little. If it was possible to paint the walls, floor and ceiling of a windowless room perfectly black, then it would be impossible to light up this room. Turn on a lamp inside it, and you could see the lamp itself, but it would seem to shine in an undefined space of utter blackness. If you yourself could stand in such a room, the lamp would light up you, but not the room. Imagine what it would look like. If you looked down you could see yourself very clearly, but you would seem to stand on a totally invisible floor, and it would be impossible to discern the walls or the ceiling or to judge the size of the room. It would be a weird experience, wouldn't it?

Okay. In such a room the light would disappear because it was absorbed by the perfect black color of the walls, floor and ceiling. But I have never seen such perfect blackness here on Earth. Have you?

So the fact that the light disappears from a windowless room can't be explained by the fact that the entire room is painted perfectly black. What happens to the light, then?

This is the answer I came up with. Light can be thought of as made up of particles that move all the time at the speed of light. The speed of light is almost 300,000 kilometers per second. (This might be equivalent to about 180,000 miles per hour, but you will have to figure that out on your own.)

Anyway. Light is constantly rushing ahead at this colossal speed. In a closed room, where everything is opaque, light will hit the walls, floor and ceiling, but it will be unable to penetrate them. Instead, it will bounce off them. After bouncing it will go back the other way, hit the opposite wall, and bounce again.And so on and so forth. Imagine a relatively big room, where the distance between opposite walls is ten meters. If light moves at 300,000 kilometers per second, it will actually bounce back and forth between opposite walls which are ten meters apart thirty million times per second!

Now, every time the light bounces off a wall, it will lose a little bit of its energy. You must remember that light is really just a tiny part of the whole electromagnetic spectrum, corresponding to waves with a wavelength between 400 and 700 nanometers. (A nanometer is 10 to the -9th meter, or a billionth of a meter.)

Every time the light bounces off a wall (or the floor or the ceiling) it loses some energy. Every time it loses some energy, its wavelength becomes a little longer. Lamplight is typically rather red, much redder than daylight, which is dominated by waves between 500 to 550 nanometers. Lamplight is dominated by longer waves, probably around 600 nanometers. And after a second, or actually much less than than a second, light that starts out at around 600 nanometers will have bounced so many times that its wavelength has become longer than 700 nanometers, after which it is invisible. And that is how it disappears: because we can't see it anymore.

Well, that is what I figured! What do the rest of you think? Woody, you are a math minor. Do I make sense? Anyone else? Rivka, where are you?

Anyway. Tough question for a six-year-old, or what do you think?

Ann