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Q & A: bending light

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Most recent answer: 10/22/2007
Q:
Hello, I am researching the properties of light for a children’s fiction story that I am trying to write (and publish). I’ve read the current information about light, but my story is based on "what if" light loses its property of traveling in a straight path (except for reflection and refraction in which light changes direction) and starts to travel like sound - in multiple directions and around corners. What I think would happen is that we would see only white light b/c all the reflected colors would bounce back to our eyes all at once producing. Since our eyes would detect all the colors, we would see only white since white light is made up of all the colors unlike white paint. That’s why I didn’t think we would see black. What do you think would happen if light started to travel like sound? Thank you for your time,
- Therese J (age 35)
Wisconsin
A:
Therese- it's always a little hard to answer 'what if' questions, because all the laws of physics tie together, or at least we think they should some day. So if you change one thing, you'd need different laws and lots of things would be different.

But anyway, working from your premise:

If you looked a particular direction, you'd see light from lots of different sources, because it could bend around to come in from the direction you're looking. Probably the colors would end to wash out, since they'd be mixtures of different colors. They wouldn't have to all become white, however, since there could still be uneven distributions of different colored objects in different directions. It sure sounds like it would be hard to see clearly, thjough. Think of how hard it is for us to judge the direction of things purely by sound.

Mike W.

Actually, light does travel just like sound in many respects. Sound travels as a compression wave in the medium (air or water), while light travels as electromagnetic waves. Waves follow Huygens's principle, and when a wave impinges on an object with holes in it or an object with a corner, the waves spread out from those places where they are allowed to pass. The general name for this is "diffraction" and you can find some answers on our web site about light going around corners.

The effect is small for very short wavelengths and very large corners, and becomes more noticeable for longer wavelengths of light. Radio waves, which are really just light waves with very long wavelengths, diffract around corners just like sound waves do. If we could see radio waves, this would be your world. Of course our vision wouldn't be very clear, because the waves would just "slosh" around people and houses and not give a nice, sharp image which can be focused.

How much light diffracts by depends on its wavelength, and so does the observed color. The reds will diffract more than the blues. This is why you see a rainbow of bright colors when looking at diffraction gratings on those metallized balloons, holograms on credit cards, and some butterfly wings and fish scales.

Light bending around corners like sound is an important limitation for people who etch transistors on computer chips using photolithography. A material called "photoresist" is deposited on a silicon surface and illuminated with light projected in a pattern shaped like the structures desired on the chip. This light changes the chemical bonds of the photoresist material, allowing it to dissolve in a chemical bath in those places where light hit it, and not dissolve where light did not hit it. Then acid is applied, which etches the exposed silicon (or metal layer that has been deposited on the silicon) but not the unexposed silicon, still hiding under the photoresist.

Transistors are currently made on silicon chips now routinely with feature sizes on the scale of a tenth of a micron. That's about one thousandth of the thickness of a human hair. The wavelength of visible light is a crucial limiting factor in how small an image can be focused on a piece of photoresist and still be sharp. Chip manufacturers now use ultraviolet light, which has a shorter wavelength than visible light, to push the feature size ever smaller.

Bacteria are experiencing exactly the world you are talking about! Of course they don't have eyes which can focus (some are light-sensitive, though), and there are other issues related to being small (like water seeming to be more viscous).

Diffraction is the big limitation for visible-light microscopes and also for telescopes.

Good luck with your book!

Tom

(published on 10/22/2007)

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