What is Light?


While sound is just a pressure wave moving through a succession of atoms and molecules, light is said to be a wave of energy which can travel through nothing: it can move through the emptiness of space, but sound waves can't because there are no atoms in space to pass pressure waves on. Now, I don't know how a wave can travel through nothing, but physicists (the experts in such matters) seem to be telling us that it is possible for light to do just that. They also claim that light is made up of particles called photons, so that makes it easier to imagine how they can travel through nothing, but it also means that they're claiming light is two different things at the same time: it is made up of particles (called photons), but it is also waves. But before we explore this puzzling contradiction, we ought to start off by thinking about where light comes from and how it enables us to see:-
Hot things such as the sun, flames and lightbulbs throw off some of their heat energy by turning it into light: they hurl billions of photons (particles of light) out in all directions. These photons travel in straight lines and keep going until they bump into things. If they hit something black, they usually get absorbed and turn back into heat, warming up the object a little. If they hit a white object, most of them will bounce off, scattering in all different directions. If they hit a gray or coloured object, many of the photons will be absorbed, while others will bounce off, and again they will scatter in all different directions. If they hit a silver object like a mirror, most of them will bounce off just as they do with white objects, only this time they won't scatter in random directions: they bounce off in predictable ways according to the direction they come from and the angle of the surface of the object, so it is possible to see other objects reflected in silver surfaces. Incidentally, silver and gold pens never produce realistic results when you draw objects which are meant to be a silver or gold color: they reflect the light falling on the picture rather than the light falling on the original objects in the picture, and while that works fine for ordinary colours, it won't show the right things reflected in silver or gold ones. The trick with drawing gold or silver objects is to try to draw the items reflected in them, giving the silver surfaces a slightly gray look, and the gold ones a yellow one.
So, when you look at an object which isn't creating new light of its own, the light that you see coming from it has bounced off it, most likely after coming from a light or the sun. Some of the photons, however, will have bounced off other objects in between, and some will have bounced off quite a few objects before reaching your eye, though the more objects a photon hits, the more likely it is that it will be absorbed before it can reach your eye, so bounced light becomes dimmer with every bounce: you can see this easily enough by walking through a house at night, starting in the only room with a light on. By the time you've gone round a few corners and into other rooms, it will be pitch black, even if every wall and object in the house is white.
Photons normally travel in straight lines: if they kept changing direction randomly as they went along, no useful images would ever reach our eyes because all the directional information would be lost along the way: We would still be able to tell how many photons are about by how bright it is, but we wouldn't even be able to work out which part of the sky the sun is in. Photons can actually drift off course, but they only do this when the medium they are travelling through changes: if they are travelling through air, but then move into a material like glass or water, they can be deflected a long way off course. This happens because light moves more slowly through dense materials, and this can change the allignment of the light waves - ah yes! Now we're now coming back to the idea of light being waves rather than particles. If one end of a wave is slowed down before the other end, that other end will race ahead of it, and this can change the allignment of the wave-front and thereby change the direction the wave moves in. When a wave of light hits a piece of glass at an angle, one side of the wave will be slowed down first, and then the middle of the wave will be slowed down when it reaches the glass, and then the other side will be slowed when it gets there: the whole wave is now aligned at a different angle from before, and this will make it follow a new course.
If that didn't make sense to you, imagine it another way: a line of people is running across a field and everyone is holding hands with the people to either side of them. The ground is hard and they are able to run fast, but the ground ahead of them is about to change to soft sand. The people at one end of the line reach the sand first because the change in surface happens at an angle across the field, so they slow down, but the people who are still on hard ground are still moving as fast as before. By the time they are all on the sand, the people at one end are well behind, but they are still holding hands, and because they all want to be running along side by side, each person simply turns round a little until the people they are holding hands with are directly to either side of them again: the line of people is now running in a different direction from before. That is how waves behave.
Lenses work by slowing light down by different amounts in different places: think about a magnifying glass and how the glass is thickest in the middle and thinnest all round the edge. A straight wave of light hitting this lens will be slowed down most in the middle and least at the edge. When it goes back out into the air on the other side it is no longer has a straight wave-front: it has become a curved "arc" which will now send the whole light wave in towards a single point of focus. This behaviour of light makes no sense if you think of it purely in terms of individual particles moving along on their own, and indeed a single photon will still act like a wave even when there are no other photons about for it to interact with. Photons must actually be made up of smaller parts which travel along as a flock, and they only pretend to be a single particle when they hit something and need to transfer all their energy into it in one single punch, but while travelling along they are more like a linked flock of smaller pieces, just like the line of people holding hands. It is not only solid or liquid things that can change the course of light: distortions in the air caused by heat changing the air pressures (and thereby the air density) can also push light off course, and you can see this by looking over a hot ring on a cooker.
There is an interesting experiment which you should know about, and it involves sending light through very narrow slits. First though, you need to know a little about how waves interact with each other. When you slosh water around in a basin, you may have noticed that when two waves hit each other they become a much bigger wave for a moment, and then they separate out again and go back to their original sizes. Sometimes you think you're sloshing waves about that don't have anything like enough energy to spill over the side of the basin, but then two waves combine for a moment and whoosh: water slops out over the edge. Waves interfere with each other: sometimes adding together, while at other times they cancel out. It's much harder to see them cancel out, but when a wave meets a trough coming the other way, and if the trough is as deep as the wave is high, the two cancel out completely and the wave and trough will both disappear for a moment as they meet up, but they spring back into existence the next moment as they move away from each other again. When light waves interfere with each other, they can do a similar thing, creating a series of bright and dark bands on a screen, depending on where they add together and cancel out.
The experiment is done using a light source, a barrier and a screen. A very narrow slit is made in the barrier so that photons can squeeze through it and travel onwards to the screen beyond. The photons can be pushed a little off course on the way through the slit, but most of them go more or less straight through, so the middle of the screen is brightest, and it gets darker towards the edges. There is no interference pattern at all. The next step is to make a second slit close to the first one so that some photons will go through one slit on their way to the screen, while other photons will go through the other. Now light shows that it is made of waves: there is an interference pattern on the screen with lots of light and dark bands. Clearly there are two lots of waves interfering with each other, one lot coming from each slit. The really interesting thing, however, is what happens when you turn down the brightness of the light source until it is only producing one single photon at a time. The lone photon travels through a slit and carries on to hit the screen. Now, obviously it won't be able to make much of a pattern there on its own, but if we put a sheet of light-sensitive photographic paper in place of the screen, we can send through millions of photons one at a time and gradually build up a picture of where they are landing: when we do this, we get a series of light and dark bands, even though there was only ever one photon travelling through the barrier at any one time: there were never two photons there at the same time to interfere with each other, and so you would not expect there to be any interference pattern, but the interference pattern is most definitely there. The fact that there is an interference pattern shows that a single photon can create this interference entirely on its own, just as if it was travelling through both slits at the same time. If you cover up one of the two slits and repeat the experiment sending individual photons through, there will no longer be any interference pattern.
If both slits are open and a light detector is placed immediately behind each slit, a photon will hit one or other of the detectors depending on which slit it has passed through to get there, so when you do this it always looks as if each photon has only gone through one single slit to get there, but this may be misleading. Each individual photon is more likely travelling through both slits at the same time, or rather some parts of it are going through one slit while others are going through the other slit. At some point, the components of the photon have to decide where to land their energy, and when they do so they have to choose a single point, so the energy from all of these components will either hit one detector or the other: you will never see just half of the energy hitting each detector. These sub-components of the photon seem to be able to communicate with each other to decide where the photon will land, and they seem to be able to transfer their energy to that point instantaneously: infinitely faster than the speed of light. Either that, or they send it backwards in time to the light source and then forwards again down the route to the point of impact. This process is not yet understood by physicists: they can't detect the small components of photons to see what they are actually doing, so we can only guess at what is happening by studying the strange ways in which light behaves. Alternative explanations do exist: there might be virtual photons travelling through the other slit to interfere with each real one, and some people actually believe real photons in parallel worlds are involved, though I tend to regard them as crackpots. Sometimes the crackpots can turn out to be right, though, so you should never rule them out entirely.
Light waves are not like sound waves in another important way: they don't travel along by pushing atoms backwards and forwards along the line of travel, but instead wobble about from side to side. If you dangle a long piece of string from your hand you can make waves travel down it by moving your hand a very short distance to the side and back at very high speed. The wave will rip its way down to the end of the string. You can make the same kind of wave by moving your hand forwards and backwards instead of from side to side: the difference then is the alignment of the wave. Light waves are normally aligned randomly relative to each other, but in some circumstances they can be polarised, meaning that they are all vibrating in the same direction. There is a special kind of filter called a polarising filter which only lets light through if it's vibrating in one particular direction. Most of the light around you is allined randomly, so a polarising filter will simply cut out most of the light and make everything look much darker, but if the light is polarised, the polarising filter won't be able to darken it at all at one angle, but when rotated by a quarter turn it will cut out all the polarised light completely. When light reflects off water at a shallow angle, the reflected light is polarised, so a polarising filter can be used to cut out all of the reflected light so that you can see down into the water instead of seeing the reflection of the sky.
Light travels much faster than sound, so if you watch a canon being fired from a long way off, you will see the smoke long before you hear the bang. While sound travels at a relatively slow 700 miles an hour, light is incredibly fast: it goes at nearly two hundred thousand miles every second (which is like going round the world seven and a half times a second). Light travels very slightly slower in air than it does in open space, and it travels a bit slower again in liquids and solids. Some special materials have recently been developed in which light can be slowed down to the speed of a snail, or even stopped completely. However, normally when we talk about the speed of light, we mean the speed of light in the emptiness of space where there is no material present to slow it down.
We still don't fully understand what light is or how it can travel through the nothingness of space, but then it isn't really true to say that space is made of nothing: there has to be a fabric to it which light waves can travel through in some way. Light waves are in fact just one of a number of different kinds of electro-magnetic waves: there are low energy waves called radio waves and microwaves, then there are medium energy waves such as infra-red light, visible light and ultra-violet light, and then there are high energy waves called X-rays and gamma rays (both of which can be used to make images on photographic film). All of these waves travel by vibrating an electric/magnetic field, so there has to be such a field spread throughout the entire universe which they are using to travel through. The big question then is, what exactly is an electric or magnetic field? I'll tell you the answer if I ever find out, but it is likely that it is some property of the fabric of space.

Try to remember the following points:-


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