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Why does sunlight shining through water drops make a
rainbow?
Whenever light moves from air into something clear
(like water or glass), it slows down just a little bit. If it hits the water
or glass at an angle, it bends as it slows down. This bending is called
refraction.
Just how much light bends when it hits water or glass depends
partly on the color of the light. The white light of the sun is made up
of all the colors of the rainbow. When this sunlight reflects off water
drops (or shines through a prism), each of these colors bends at a slightly
different angle, fanning out to make a rainbow.
Why do I have to stand with my back to the sun to see a rainbow?
To make a rainbow, sunlight has to shine into a
raindrop (bending as it moves from the air into the water), bounce off the
far side of the drop , and then leave the drop (bending again as it moves
from the water to the air). So if you're going to see a rainbow, you have
to be standing where your eyes can intercept this colored light. That means
you have to stand with your back to the sun, so that the sunlight is shining
into the raindrops from over you shoulder.
Knowing this helps you know when to look for rainbows in
the sky. You're likely to see a rainbow when the sun is low in the western
sky and sunlight is shining on raindrops to the east. At the same time of
day, you won't see a rainbow in raindrops that are west of you. That's because
you're facing into the sun.
What else can I see when I'm looking for rainbows?
If condition s are right, you may see two rainbows--one
inside the other. The inner, brighter arc is the primary rainbow; the dimmer,
outer arc is the secondary rainbow. Notice the order of the colors in your
rainbow. In the primary rainbow, red is on the outside of the arc and violet
is on the inside. In the secondary rainbow, the colors are in the same sequence,
but violet is on the outside and red is on the inside.
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Stand near the spray of water drops. Stand with
your back to the sun. Find your shadow. It should be between you and the
spray. If it isn't, move around until it is.
If you don't see a rainbow, here's
how to find one. Look for the shadow of your head. Hold both arms straight
out in front of you. Spread your hands as wide as they will stretch with
your thumbs touching, tip to tip. Place the tip of one little finger so
that its shadow falls in the center of the shadow of your head.
When you move, the rainbow moves with you. Someone
standing next to you may also see a rainbow in the spray, but their rainbow
will be in a slightly different place. Each person sees his or her own personal
rainbow.
Look for a rainbow in the spray.
