Lois & Clark Forums Forums General Discussion Off Topic Where does the light go?

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

Trying to think like a six year-old, this would be my answer:

You know how a light bulb is made, right - there's the globe and then there's the metal base.

Well, inside that metal base, there's a miniature vacuum. When the light is off, the vacuum is accumulating light particles from the room, or the outside, or wherever the bulb happens to be. These are so infinitely small, of course, that even if it's accumulating them all day, the metal base never overflows. And then when you turn the light on, the vacuum turns into reverse mode and it spits out the light back at you, so you can see clearly.

Therefore, when you turn the light off, the light particles are sucked back into the base, through the vacuum. And that is where light goes.


*giggles* I love how kids think... I like to write fairy tales and children stories exactly for that reason - because you can literally take any object and give it a life of its own or find some wonderful and magical way of explaining it. The universe is full of cool, magical mysteries to them. Sometimes I wish it still was for adults, too...

Actually, black body radiation ... is pretty strange and mostly irrelevant here. Never mind.

If you were to seal off the room and make all four walls perfectly mirrored, then the light would actually stay around. Except, of course, that you'd be in the room, and absorbing some of the light yourself.

Otherwise... anything less than a perfect mirror will absorb some of the light that hits it. A photon hitting an electron will be absorbed into that electron, energizing it. That's how plants live. Whenever a photon (preferably moving at a wavelength in the green area of the visible spectrum) hits an electron orbiting a certain atom that's part of a certain molecule, it gets "charged," and the plant can use that to help make food for itself. (Which just blows my mind. For the whole thing to work, that one exact atom has to be hit over and over, and it has to happen in enough of those molecules every day to feed not just the cell that it's in, but also those parts of the plant that aren't capable of photosynthesis. It has to happen enough in the leaves of a tree to feed the entire trunk and all the branches, with enough energy left over to last the winter, when there are no leaves... All from photons from 93 million miles away hitting exactly the right atoms.)

Anyway, back to the subject at hand...

Light comes out of the bulb and bounces off a wall.

Some of the photons will pass through the surface. If you hold a piece of paper up to the light, you'll see that, even though it's pretty much opaque, at least a little light gets through. So some of the light is lost that way.

Some of it gets absorbed by the paint (which briefly gets slightly warmer).

The rest is reflected off the wall. That percentage can vary, but let's say that this wall reflects 70% of light that hits it. 30% was absorbed (including the small fraction that passed through the surface to be absorbed deeper). The rest bounces on... to the next wall, where another 30% is absorbed. Then it bounces again, and another 30% is absorbed. And so on.

Some will also go out the window and through the cracks and all. But even if there were none, the walls would eventually absorb (pretty much) all of it. As you noticed, at the speed it's going, it won't take long at all for that to happen, even if it takes a thousand bounces.

You, of course, will also absorb some of it. Into your eyes, where the information will be transmitted to the visual cortex. But also into your skin and clothes. And any other objects in the room. Including the lightbulb itself, which isn't perfectly clear or mirrored, either.

As for what a six-year-old would say... I'd be interested in hearing it. I'd guess at least a few of them would assume that it flies out the window. Enough bounces, and it will eventually find any crack.

Seems to me that this was a misleading, confusing, and even irresponsible question to pose to a six year old, particularly one with learning difficulties. To ask, 'where does the light go?' when you turn off a lamp implies that the light does indeed go somewhere, when in fact, it doesn't go anywhere at all. It's quite simply the wrong question to ask - there's no sense in which the light itself does anything at all. You turn off the lamp and you remove the only source of light in the room. End of story.

Yvonne

The question is about the photons that were left bouncing around the room just before the light was turned off.

Those do have to go somewhere. In a perfectly mirrored room, they would continue to bounce around until you yourself absorbed them all.

It's kind of like a little problem I used to wonder about... If you take a box outside and close the lid, at least some of the photons should be trapped inside (reflected off the closing lid back into the box). And yet, inside the box, once you close the lid, it's dark.

Actually, that was the subject of a Chelm story, where the villagers try to capture light in barrels to store for the winter.

Of course that doesn't work, but I always wondered what would happen if you tried that with a mirrored box.

But then, even if it did work, all the light would escape (at the speed of light) as soon as you opened the lid, so that wouldn't really do too much for you anyway.

All of which goes to show you that the best way to store light is in a solar-powered flashlight. (Which, joking aside, actually does exist... You charge it with a solar cell during daylight so that you can run it off of battery power at night.)

Ann asked:

Yes.

Many years ago, my parents took us on a trip where we walked deep into the earth in a cavern in Arkansas. At one point, in a 'room' deep in the cave where there was nothing to bump into (if we didn't walk around) and no place to fall, the guide turned off the light at told us that we were too far underground to see anything without artificial light. Our eyes would never - never - adjust to the reduced light level because there was absolutely no light in the cavern.

He left the lights off for about twenty seconds and everybody got very uncomfortable. Unless you've experienced total darkness, you don't know what it means. And I don't have any way to explain it any better than that. I don't think there's any frame of reference for it, unless a person is already completely blind.

To respond to Yvonne's comment:

Maybe that's what the teacher was trying to get these kids to understand, that light doesn't exist without a source and that it doesn't "go" anywhere when you turn off a lamp. Since I've never taught that grade level, nor do I have any training in teaching special students, I don't know if the question was out of order or not.

But it sure got my attention.

Not at all how a 6 year old would think, but I'm actually a little intrigued by this - mind if I have a crack? (Bit silly to ask, really, since I'll have posted by the time you're reading this anyway...)

TOC said that light is a photon - a discrete packet of energy, which is absolutely true. Thing about our eyes is, they can't recognise a single photon of light. Light, thankfully, is also a wave - sort of like a photon beam - and when we perceive it, our retinas are bombarded with lots of these photons in succession. Which is great - yay! - cos this means we can see.

So, we need light to be a wave to see it - thing about light waves, though, is they abide by the 'Inverse Square Law' which means, as they move further and further away, they spread further and further apart - this reduces the intensity of light (lots and lots of the photons in a small space = very intense, same number of photons spread over a much larger space = not nearly as intense). This isn't good in terms of the succession of photons thing that we need so we can see.

More so, every time they bounce against a wall, not only are we moving over more distance and becoming less intense, the 'bounce' off the wall isn't perfect. (Think of a stream of water, shooting out of a hose and hitting the floor - it doesn't bounce back perfectly, it scatters and goes everywhere. Same thing happens with light.) Which, again, is not good for the succession of photons we need so we can see.

Between the scattering of the photon beam, the rapid drop in intensity and the speed at which this happens, light appears to vanish when we cut off the source.

Dave (who's apologetic for how long that was)

Yvonne, I need to clarify two things. First, that the six-year-old in question had no learning disabilities whatsoever. (His father teaches teenagers with learning disabilities, but there is nothing wrong with the son.) Second, that the assignment for the pre-schoolers was not to find the correct answer to the question, but to discuss it with his or her parents and to come back next day and tell the other children about the discussion that each of them had had with their parents. I suppose the idea was to make children wonder about things and make the curious about the world around them. Of course, if the children came back next day and nobody knew the answer to the question, and the teachers at the pre-school couldn't answer it, either - which is a distinct possibility! - then I don't think it was a very good assignment to give to young children. Clearly, there is a correct answer to this question, and I don't think that teachers should encourage children to think that any answer you can come up with to a question is as good as any other.

Ann

P.S. Thanks to everyone else who commented! Thanks for the story about the cave, Terry! Me the claustrophobiac would have had a complete nervous breakdown down there. I still want to point out that the blackness you experienced down there was not caused by the fact that the cave surrounding you was perfectly black in itself. As soon as the guide turned on the light again, I'm sure you could see the walls of the cave. If the cave had been perfectly black and absolutely unreflective (which is what perfect blackness is!), you could not have seen the walls even if the guide had shone his flashlight at them. But thank you for telling me about the cave... reading about the total blackness there gave me the shudders.

And Dave, thank you for explaining the scattering effect as the wave of photons bounce off the wall! And a very big thank you to Paul, too. You're right, Paul, every time the light hits something some of the light will be absorbed. After a sufficient number of bounces, all of it will be absorbed, of course!

Oh, P.P.S., Lara, I loved that explanation about the metal base of the light bulb!