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Dr WLS
Dr WLS, PhD in physics and engineering
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Physics

Customer Question

Radio waves are able to diffract readily around buildings, as anybody with a portable radio receiver can verify. However, light waves, which are also electromagnetic waves, undergo no discernible diffraction around buildings. Why not?
Submitted: 11 years ago.
Category: Homework
Expert:  Michael replied 11 years ago.
The problem is kinda with your question. Waves of energy, radio, micro, x-rays all pass through things and are stopped by other things. Lead stops most rays but not some other high energy waves. Radio waves are pretty good at getting through some things, but you may notice that am radio waves travel farther yet drop out while you may pass under a bridge. While fm radio waves pass through the bridge but don't travel as far, or more accuratly don't follow the curature of the earth.

Light is a problem, it acts as a wave sometimes and as particules of energy at other times. But for now lets treat them as waves. They do pass through some things, mainly glass. but the also reflect (mirrors).

This all brings me to my favorite wave. the micro wave which in fact not micro at all compared to other wave types. Next time you cook in the microwave oven look at the food through the window. you will notice that you are actually looking through a mesh of tiny holes. these holes allow the light through but not the larger micro waves.

I hope that helps.

Just for fun: if you are riding in a car and driving 60 mph towards a the radio station you are hearing the music 60 mph faster than if you are standing still. your brain can't detect the difference it or compensates for the change, so you don't notice. Just a bit of doppler effect radio style.

Customer: replied 11 years ago.
Reply to Michael's Post: Michael,

I'm not sue I understand how your answer explains why light waves do not diffract around objects such as buildings the way radio waves do. Please clarify.

Thanks,
Rob
Customer: replied 11 years ago.
Follow up: Here's an explanation I found listed on a physics site... http://www.madsci.org/posts/archives/nov2001/1004649219.Ph.r.html

Waves can bend around corners because of an effect called diffraction. The amount that a wave will bend around a corner because of diffraction is about the same as its wavelength. The wavelength of a typical sound wave is a few feet. On the other hand, the wavelength of light waves that you can see is about a million times smaller than this. So it is much easier to see the bending of the sound waves, because it is about a million times bigger than the bending of the light waves.
Expert:  xarqi replied 11 years ago.
The explanation that you have found describes the relationship
between wavelength and diffraction, which is the essence of the
answer to your question.

It looks like you've answered your own question. Do you need
any more input on this?

( Note that the phrase "a million times smaller than" as used in
the quote you gave is meaningless. The correct phraseology is
"a millionth of".)
Expert:  Dr WLS replied 11 years ago.
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