On understanding waves

Xander wrote a post on How Not To Make a Faraday Cage where he experiments with shielding his oscilloscope from the interference of the electricity running all through his house, and I found this astoundingly clear way to think about shielding from waves, Like how to understand the way the mesh grid on the door of your microwave oven is designed to stop the microwaves from getting out.

by commenter “Matt”

The size of screen you need to make a cage depends on not only the wave length, but the magnitude of the waves as well. The idea is to force any incoming wave through a “tunnel” in order to get to the inside of the cage. By making the hole small enough and the wall thick enough, you can force a wave to “crash into the tunnel wall” and be absorbed.

If that doesn’t make much sense, try this. Take a spare piece of wire and bend it into a sine wave. Then take a stack of washers (all the same size) and try to feed the wire through the center of the stack. Notice that if you make the hole smaller, it’s more difficult to thread the wire without losing the wave. If you make the stack of washers larger, the same thing happens.

Practically speaking, a sheet of shielding material with holes of 0 diameter is 100% effective (assuming the shielding is thick enough to absorb the energy of the wave. Tin Foil doesn’t stop nuclear blasts and all…) The only purpose of adding all the holes is reducing the amount of material used. That makes it cheaper to construct and easy to manipulate the shield material (think window screen vs. a sheet of solid copper).

totally cool.  I love clear visualization!  I’m adding it to my arsenal of ways to explain electricity clearly and simply to myself and to others.


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