This week's experiment is
something that was noticed by my good friend and cohort James Ashley. We were on
our way back from meeting with our FPL contacts in Ft. Lauderdale and
were driving through some rain. After the rain stopped, James noticed a
wonderful rainbow, but also noticed that part of it was missing. This was
strange, as I could see the entire thing. Very quickly we found that the
difference was in his sunglasses. To try this, you will need:
polarized sunglasses a rainbow
If you don't have a natural
rainbow, you can make one with a garden hose on a sunny day. Stand with the sun at
your back and make the water spray as fine as possible. Adjust the angle
until you see a nice, bright rainbow.
Once you have a nice rainbow,
then you are ready to make it disappear. Put on your polarized sunglasses.
Look at the rainbow again and while you are looking tilt your head left and
right. As you tilt your head, the rainbow will either get brighter or it
will vanish. Hold the sunglasses in your hand and turn them in a circle. You
will notice that as you turn the lens, the rainbow will get brighter, then
dimmer, then vanish and then reappear to get brighter again.
What is happening? As the
sunlight is broken up into the rainbow, the light is polarized. What does polarized
mean? As a beam of light moves forward, it also vibrates from side to
side. A nonpolarized beam of light vibrates at a 90 degree angle to the
direction that it is traveling. As the beam travels, some of the light is
vibrating up and down, some if it from side to side and all of the angles in
between.
If the beam is polarized, then
the light is only vibrating in one direction. You are seeing polarized light
when you look through polarized sunglasses. These sunglasses filter the
light, only letting through the light that is vibrating in one direction. Light
can also be polarized by refraction, being bent as it passes through a
substance. This is what happens with the rainbow. As the sunlight is
separated into the different colors, it is also polarized. The glare reflected
from water is also polarized, which is why sunglasses can help fishermen see
the fish. They filter out the polarized glare, letting you see into the
water.
How does this explain the
vanishing rainbow? Imagine the light from the rainbow polarized so that it is
only vibrating up and down. Now imagine that you are looking at it through
polarized sunglasses that filter out the up and down vibrations, only letting
through the light that is vibrating from side to side. In that case, the
sunglasses would filter out the rainbow, and you would not see it. If you turn the
sunglasses 90 degrees, then they would let the rainbow light through and it
would reappear. Since all of the light from the rainbow is coming through and
only part of the unpolarized background light is coming through, the
rainbow will appear to be brighter through the sunglasses than it is without
them.
There are lots of other
experiments that you can do with polarized sunglasses. If you have two
polarized sunglasses, place one in front of the other and turn one of them. You
will see the lenses get darker as you turn them, with the two crossed
polarizers filtering out almost all of the light. Continue to turn them and they
get lighter again as the two polarizers line up.
Back
to Krampf Index Including permission to post
these
experiments on my web site.
