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Krampf #477 A Different Rainbow
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KRA-@aol.com
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Feb 28, 2007 20:22 PST
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Robert Krampf's Experiment of the Week
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This Week's Experiment - #477 A Different Rainbow
Greetings from our home in Jacksonville. After so much time on the road, it
is great to have some time at home. I have been enjoying cooking, catching
up on reading, flint knapping, and doing some writing.
Tomorrow I begin a new project, which you should find interesting. We are
going to make video versions of some of the past experiments. I am starting
with some of the early experiments that many of you never saw. From those, I
am selecting the ones that I think will work best on video. It will take a
while to tape and edit the first ones, but we should have the first one up
soon. We will also be posting some of my other science videos, so everyone can
have them free of charge. I will let you know when and where they are posted.
This week's experiment is one that you have probably seen a thousand times,
but you might never have noticed what you were really seeing. To try it, you
will need:
a glass of water
detergent
a straw
Fill the glass about half-full of water. Add a drop of dish washing
detergent, and stir with the straw. Then blow gently through the straw to blow some
bubbles. Hold the glass near a light and notice the colors in the bubbles.
OK, the bubbles have a rainbow of colors. You have probably seen them many
times before. Now look carefully at the colors. Then think about the last
time you saw a rainbow. Do you remember the colors of a rainbow in order?
Do you remember Roy G. Biv? That is the way most people remember the colors
of the rainbow. Red, orange, yellow, green, blue, indigo and violet. The
physics of a rainbow dictates that the colors will always be in that order.
Now, take another look at the colors of the soap bubbles. The colors are
different, and they are in the wrong order. In the bubbles, the colors are
yellow, magenta, blue and blue-green. Why are they different?
It has to do with the way the colors are formed. With a rainbow, the colors
are caused by separating the different wavelengths. Each color is made up
of light waves of the same length. The red you see is made up of light waves
in the red wavelengths. The colors are also in order, from the longest
visible wavelength, red, to the shortest visible wavelength, violet.
In the soap bubbles, the colors are formed in a different way. Here, the
colors are produced by removing colors, not separating them. When light hits
the surface of the bubble, part of it is reflected and part of it passes into
the liquid. When it hits the inside surface of the liquid, again, part of it
is reflected back and part passes on through.
Here is where it gets interesting. The light that reflects back from the
bubble's surface mixed with the light that reflects back from the inner surface.
Because of the thickness of the bubble, the waves of light are out of step
with each other.
Whether we are talking about light or water, waves act the same way when they
meet. Think about waves in water. You have the high part of the wave,
called the crest. You also have the low part of the wave, called the trough.
If two crests meet, they combine to make one very high wave. If two troughs
meet, they combine to make a very deep trough. If a crest and a trough meet,
they cancel each other out, and you get no wave.
Now, lets go back to our bubble. If the bubble is the right thickness, the
red waves of light will be just enough out of step so that they will cancel
out the red light. The other colors have a different wavelength, so they are
not canceled. If you remove the red light, you are left with a blue-green
color. If the bubble is a little thinner, then you will cancel out the yellow
light. That gives you a blue color. A little thinner and you cancel the
green. That leaves a color called magenta, a mixture of red and blue light.
Even thinner and you cancel the blue, giving a golden yellow color. Make the
bubble a little thinner and the cycle starts again, so you get repeating bands
of blue-green, blue, magenta and golden yellow.
If you watch the bubble, you will notice that the colors swirl and shift.
That is because gravity is pulling the water in the bubble downwards. The
bubble gets thinner as it gets older, and the top is thinner than the bottom.
Once you know these colors, you will start to see them other places. You
can see them in the "rainbows" that you see in an oil or gasoline film on water.
You can also see them in the iridescence of sea shells and insect wings.
I tried thinking of a way to connect this with ice cream, but the only thing
I could come up with was eating a bowl of rainbow sherbet. Ahh, the things I
do for science.
Have a wonder filled week!
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Robert Krampf's on-the-road schedule:
My calendar is now on-line. You can see a detailed calendar, with dates,
times, schools, etc. at http://www.krampf.com/tour.html
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Check out my web site at:
http://www.krampf.com
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
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