This Week's Experiment - #258 Dark Lines
This week, we are going to take another look as something that we looked at a
few weeks back in #252 Bubble Colors. Then we saw how light waves could
cancel each other to produce the colors we see in a soap bubble. In that
experiment, we only canceled out part of the light, removing some colors so
we could see others. This time, we are going to cancel it entirely! You
will need:
your fingers
a bright light
Sit a few feet away from the light. Hold your hand up in front of your face,
with your fingers held out straight. If you look, you will see that there
are open gaps between your fingers. Start with your hand about an inch away
from your eye, looking at the light through one of these gaps. Slowly move
your hand away from your face. You should begin to see dark stripes or bands
inside the gap. If you have trouble seeing them, squeeze your fingers closer
together or move them a little apart, until they are easy to see.
These lines are called diffraction fringe. What causes them? As the light
moves through the slit between your fingers, it bends and begins to spread
outwards. This is called diffraction. As the light spreads, the light waves
meet each other and something wonderful happens.
Picture waves in water as they meet each other. Each wave has a part that is
up (the crest) and a part that is down (the trough). If both waves are up
when they come together, they add their heights and you get a very high crest
as they meet. If both are down, you get a very deep trough. But what
happens if one is up and the other is down? They cancel each other out and
you get flat water. In this case, instead of flat water, you get dark areas.
This experiment is based on the work of Thomas Young. He was an English
scientist who in 1820 used a similar experiment in his study of light. Light
is a strange thing. In some ways, it acts as if it is made of tiny
particles. In other ways, it acts as if it is made up of waves. This
experiment is an example of light acting as waves. If two particles hit each
other, they don't both vanish. The only way that Thomas Young could think of
that two beams of light could meet and cancel each other out was if light was
made up of waves.
In Young's time there were bitter arguments between scientists, with some
saying that light was made or particles and others saying that light was made
of waves. Now we take a much more practical approach. We say that light is
made of photons, which can act like particles in some ways and like waves in
others. That leaves more time to argue about other puzzles in science.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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