MadSci Network: Physics

Re: wave diffraction and wavelength

Date: Wed Mar 1 22:22:53 2006
Posted By: Zack Gainsforth, Undergraduate, Physics, U.C. Berkeley
Area of science: Physics
ID: 1139812331.Ph

As you know, light is a wave.  It is the fact that light is a wave that
causes it to make a diffraction pattern.  The best way to understand why
this works is to do a similar experiment with water in a bathtub.  First
fill up a bathtub halfway and place two wooden boards vertically side by
side with a small gap in between.  The idea is to block water from moving
from one side of the tub to the other, except via the small gap, which
should be a couple of inches wide.  Plywood also works great.  Then you can
tap the surface of the water with one hand to create waves.  You can also
try gently sloshing your hand back and forth.  As the waves go through the
opening in the wood, they will create exactly the same types of patterns
that light does.  The only difference is that the water will have a
wavelength of a few centimers, whereas light has a wavelength of only a
half of a thousandth of a thousandth of a meter (really small!)  If you tap
the water faster, you will see that the wavelength of the water is shorter.
 If you tap the water slowly, then you will see that the wavelength of the
water is longer.  Then, what you do is you look at the water pattern that
bounces back from the far end of the bathtub.  You will see a diffraction
pattern.  It won't glow like light does, but it will have a similar kind of
shape -- at least until the water bounces around the tub a few times and
the waves get confused.

If you do this experiment for a bit, you will notice that the opening is
spreading the wave out.  But the pattern that is generated depends not only
on how the wave is spread out, but also the shape the wave had before it
spread.  This is because the pattern is caused by some parts of the wave
"interfering" with other parts.  That is, at one point the wave is higher
than the other.  When those two parts touch, then the wave disappears.  But
when two high points touch, they make a very high point.

Now, what you want to do is to use something that vibrates.  You can fill a
pot up with water and place it in the bathtub so the lip is above the
surface of the water, but it sits squarely on the bottom.  You may have to
try a few pots -- generally ones that make a big "gong" sound when you hit
them work better.  Then, once the pot is in the water, and the waves have
had time to settle down, rap the edge of the pot with your knuckle.  You'll
see a new pattern of waves sent out.  These waves are generally much
smaller than you could've created with your hand.  If you look to see what
diffraction pattern they make, you will see it is different than from your
hand.  You will have to do the experiment to see what pattern you get,
because it depends on how you position your boards, and what sound your pot
makes when you hit it.  The point is that the pattern you get when tapping
the water with your hands is different than what you get from the pot.  The
gap between the wooden boards is the same, but the shape of the water wave
has changed.  Once you've conviced yourself that this really does change
the pattern, you can start to experiment with different types of waves to
see how the pattern depends on the wavelength.

In fact, the only time you really get to see a diffraction pattern is when
the wavelength is similar to the size of the gap.  If light had a
wavelength that was meters long, the diffraction pattern through a tiny
opening would theoretically exist, but you wouldn't be able to observe it.
 Essentially no light would go through the hole.  Likewise, if you try to
diffract light through a hole that is, say a foot in diameter, you won't
notice any diffraction pattern except along the edges.

I hope that clarifies things a bit!

[note added by MadSci Admin:  You can find many previous answers in our
archives discussing this by using our search engine
( to search on 
"diffraction wavelength".]

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2006. All rights reserved.