QUESTION #509

# Is it possible to slow light down?

The short answer is No. Einstein's theory of special relativity is based on the idea that the speed of light is always constant. However, we CAN make it take longer for light to travel a set distance. In fact, we say that light travels more slowly in optically dense media. That statement is somewhat misleading. We need to look into the physics of the phenomenon.

When light enters a material, photons are absorbed by the atoms in that material, increasing the energy of the atom. The atom will then lose energy after some tiny fraction of time, emitting a photon in the process. This photon, which is identical to the first, travels at the speed of light until it is absorbed by another atom and the process repeats. The delay between the time that the atom absorbs the photon and the excited atom releases as photon causes it to appear that light is slowing down.
Answered by: Gary Russell, M.S., Systems Engineer, Reston, VA

Yes and no. If you're talking about light traveling through empty space, then no, there is absolutely nothing we can do to slow it down. (In fact, special relativity says that even if we run away from a light wave really really fast, it will still be coming towards us with the same speed. If you haven't heard of or don't understand special relativity, don't worry too much about this note.)

As light travels through matter, though, it "bumps into" the atoms (technically, the photons keep getting absorbed and re-emitted), and so the light _appears_ to travel slower. We measure this phenomenon with a number called the index of refraction, usually represented by the variable 'n'. N is defined to be the speed of light in a vacuum divided by the slower speed in matter, and it depends both on the type of matter in question (different atoms/substances absorb and emit light in different ways) and the wavelength of light in question (different wavelengths of light get absorbed and emitted at different speeds, even in the same substance).

T.M.I. (Too Much Information):

This apparent slowing of a light wave is responsible for the way light bends as it enters mediums like glass and water. A cleverly shaped piece of glass (or other material) can take advantage of this property and bend groups of light rays to make an image appear larger or smaller. Such pieces of material are commonly called lenses, and are used practically everywhere.

Prisms work because of the fact that different wavelengths of light have different speeds in the material, and so get bent different amounts. The different colors composing "white" light get bent differently as they go through a prism, and the resulting separated colors form the rainbows you generally see coming out of prisms.

[ I am going to answer this using a traditional electromagnetics viewpoint as light as waves; this can also be answered with QED, quantum electrodynamics, which has a much different model; at our level of observation though, I see no need to get that detailed, and this wave explanation should hold very well; as an electrical engineer, even though I understand QED, I use emag in my everyday life ]

[ That being said, I am also going to leave out the ideas of conduction, attenuation, and dielectric absorption; I see no reason why these would help answer this question ]

Yes, light "slows down" all the time through different materials other than vacuum. Light's speed in air is nearly the same as its speed in vacuum, so we just consider its speed through air to be the same as its speed in vacuum, c (around 3e8 m/s).

Light is an electromagnetic wave, and the materials through which it travels make up a transmission line. This transmission line is very similar to the ones which carry network signals from computer to computer or TV signals from cable companies to cable customers. In fact, the broadcast of TV signals is very much the emission of low-frequency light through a transmission line extending through the air from the antenna to your TV.

Every transmission line has capacitive and inductive effects. You may have heard of electrical permittivity, which is in many ways a measure of the capacitance of a material, and electrical permeability, which is in many ways a measure of the inductance of a material. Every material has these two characteristics, and they are what governs the speed of the wave (and the speed of light).

From a circuits point of view, you can picture a transmission line like a number of inductors put end-to-end with capacitors connecting each point between each inductor to the return wire of the transmission line. A signal propagates by charging each inductor up, discharging each into the next capacitor, and taking that capacitor's charge and discharging into the next inductor.

A more physical example might be an ice tray. If you take an ice tray and tilt it at an angle and start to fill up the empty cube nearest to the top, the water should gradually fill up that cube, start to leak into the next, and eventually do this to the next one down.

There are a number of other ways to visualize this -- perhaps a slinky-like effect down a transmission line. But in each it is obvious that it is going to take time for the energy put in at one end of the line to hit the other end of the line, and by changing things like the size of each capacitor and inductor (or perhaps the size of each ice cube), you change how fast the wave can propagate down the entire line.

In fact, there are relationships between the "characteristic impedance" of a material and these two characteristics as well as the velocity of propagation of a material and these two characteristics. These relationships (for a very good reason) mirror the relationships we see between the inductance per unit length and the capacitance per unit length of man-made transmission lines and their own characteristic impedance and velocity of propagation.

In materials like glass, the electrical permittivity is greater (2.554 times greater is a number that comes to my mind) than that of air. As a consequence, each little "capacitor" takes more time to charge, and the light that travels through the glass takes a longer time to move from one point to another. You can actually see this happening with "refraction." Whenever you see light bending when entering a material like glass or water, this is a result of the light slowing down as it enters.

For example, imagine sliding on a piece of rectangular plastic (perhaps a cafeteria tray) on a flat piece of ice that makes a large rectangle like a rectangular skating rink. Imagine that that ice butts up to pavement which is easy to walk on all sides. Now imagine being flung toward one of these ice-pavement interfaces at some angle that is not perpendicular to the interface. What is going to happen? I think you'll find that once a portion of the tray hits the pavement, it is going to slow down much quicker than the rest of the tray and cause you to swing around so that the tray's propagation is much more perpendicular to the interface. That is, while you may have expected to come out very close to the side of the ice rink, you may find yourself facing away from the ice rink heading off at a much different angle.

This is what happens when light moves from air (like ice) into glass (like pavement) at an angle. You can picture the "front" of the light like a wall (this is due to the tangential and perpendicular components of the wave, but I don't want to get into that much detail) which is being moved toward the interface. As the side of the wall nearest to the glass hits the glass, it slows down and the rest of the wall in the air keeps going. This causes the air side to swing in toward the glass. Eventually the entire wall will be inside the glass, but will be headed at a very different angle. The interest thing is that if the glass is a flat thick slab, when the "wall" hits the other side of the slab and hits air again, it will bend back to its original angle; however, if you drew a line from its new path toward its old path, you'd find that they are parallel yet not the same path.

You also observe this in a pool. The light is slower through the water, which causes it to bend, and causes you to see things at angles which they are not.

This also happens on the road. You may often notice that the road seems to "reflect" light far ahead on hot days. This has to do with the permittivity of the air just above the road (very warm air) and the permittivity above that air (cool air). This involves a more complicated explanation, so I won't get into it, but I provide it here as something for you to think about.

And that brings me into another topic of interest -- reflection. You see reflection of light all the time -- now you known a little bit about why it happens. Reflection is caused by differences in permittivities and permeabilities from interface to interface. This is a very in-depth topic that I won't get too far into. It involves reflection coefficients, transmission coefficients, things like standing wave ratios, and a number of other things to help characterize what happens when waves travel from one medium to another.

The interesting thing about this though that you may not have thought of (or, if you're a stereo enthusiast, perhaps you have) is the idea that since light is an EM wave as well as any signal on any man-made transmission line, and light suffers from reflection going from one set of permittivities and permeabilities (which make up characteristic impedances) to another set, signals on our own man-made transmission lines also have these sort of reflections. In fact, one thing that antennas do is help to make a man-made transmission-line's characteristic impedance the same as mother nature's transmission line so that the energy in space can be brought inside a man-made transmission line to be processed. And once on this transmission-line, the impedance has to be carefully taken care of. For example, in an FM di-pole antenna, there is a T junction at a particular spot that not only helps to eliminate changes in impedance from the legs of the T to the body of the T, but also helps to only pick up FM signals.

Anyway, this is a more complicated topic. However, if you've ever noticed that your speakers are rated with a certain impedance (4 ohms or 8 ohms, for example), and that perhaps your receiver also has a way of changing the type of speakers it expects to drive, and perhaps you've noticed your speaker cable has a certain impedance as well . . . Well, all of this has to do with reflection and maximizing power output at the speakers, and these same principles that govern how the data from your receiver hits your speakers also govern how light hits glass.

Though, I suppose technically nature governs all natural interactions whether electromagnetic or not, so of course the same thing governs both. It's just a very close relationship between those two.

So, yes, light is slowed down when traveling through different materials which have different characteristics which affect how it is transmitted through them.

You may want to check out a recent article:

http://www.eet.com/at/news/OEG20020826S0041

Which talks about how scientists are now able to actually "speed up" light and cause it to behave in exactly the opposite way that we usually expect.