Light From Small Sources Spreading Out

Most recent answer: 10/22/2007

Q:
Light Why do light from a small light source spread out in all directions?
- Anonymous (age 13)
Toronto, On
A:
This is a very very interesting question and touches on some of the history of our knowledge of light and also on the technology of using light.

Light can be though of either as composed of electromagnetic waves or as a collection particles (photons), and it behaves as either depending on the circumstances, but really is a bit of both. In this case, the wave nature of light is the most important.

If you’re asking about light sources like flashlight bulbs or LED’s, these are still too big for the size of the source to be interesting. You can get light from these sources to all go in parallel directions with mirrors and lenses. But if the whole light source (mirrors, lenses, and all) has a size that is comparable to the wavelength of the light, then the light source will emit light in all different directions and it will be impossible to keep this from happening. Typical visible light (green) has a wavelength of about 5000 Angstroms or about 0.00002 inches. Radio waves have much longer wavelengths and the effects of "small" sources are much more apparent with larger sources than with visible light.

The reason for light coming from small sources in all directions is the same reason that a stone dropped in a pond will create a round wave going off in all directions with equal strength, no matter what the shape of stone is, as long as it is small compared to the wavelength of the ripples on the pond. If instead you put a large flat sheet sideways in the same pond and shook it back and forth, then you can get long, straight waves all going in the same direction. You can get roughly the same effect by dropping many many stones in the pond all at once and in a line. A beam of light traveling in one direction (as opposed to spread out in all directions) consists of train of waves all lined up and all traveling in one direction.

Christiaan Huygens studied this phenomenon back in the 1600’s and described why light must be described by waves. Each wave can be thought of as a sum of very many smaller waves all propagating in all directions. But if all the smaller waves are lined up just right and in time, then the sum will be linear waves as needed. Here is a web site with a picture or two of what’s going on.

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In particular, notice that when a beam of light all going in one direction strikes a surface with a small hole in it has on the other side light propagating in many different directions. This phenomenon is called diffraction. In effect, the small hole is a small light source for the other side, and it is incapable of reproducing the wave train behind it because of its small size -- it can’t let the whole wave train pass, just a little tiny bit, and that tiny bit will go in all directions by Huygens’ principle.

We have some Physics Van questions and answers on this topic too:





Diffraction has tremendous techological significance. It limits how small an object can be in order to be seen with a microscope. It requires some radio stations to have many towers lined up in a row in order to make a more flat wavefront to be directed at a city rather than being radiated uniformly throughout the countryside. A single radio tower is a "small light source" when compared to the wavelength of radio waves. It means that radio telescopes and optical telescopes have to be gigantic in order to see faraway objects that are angularly close together. (The bigger the wavelength, the bigger the telesocope has to be, and the less separation between objects to be looked at, the bigger the telescope has to be). It even means we have to build big particle accelerators to investigate the physical reactions of subatomic particles, because we need a short-wavelength probe to investigate very small objects.

On a more relevant technological scale, in order to etch small transistors on silicon chips to make better and faster computers, a limitation is the wavelength of the light and the size of the smallest spot that can be illuminated. (Light works in reverse -- just as you cannot make a small source emit light in one direction, so too you cannot have light coming from one direction focused down to a small spot if the wavelength is comparable to or bigger than the size of the spot). So chip manufacturers have been forced to change the light they use to print the designs on chips from ordinary visible light to ultraviolet light. They have thought about using x-rays too, and electrons, but these aren’t yet commercially viable.

Tom

(published on 10/22/2007)