Rainbow
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Rainbow
The most charming example of chromatic dispersion is a rainbow.
When white sunlight is intercepted by a drop of water in the atmosphere.
Some of the light refracts into the drop, reflects from the drop's inner surface,
and then refracts out of the drop.
As with the prism, the first refraction separates the sunlight into its component colors,
and the second refraction increases the separation.
And the rainbow is there in the sky. This applet shows the physics of rainbow.
The black circle represents a drop of water in the atmosphere. Red light coming from the left.
50%| and 50%+ means the incoming ray is unpolarized.
And we can mathematically transform unpolarlized light into the superposition of
two polirized waves whose planes of vibration are perpendicular to each other.
In this case, 50%| in the up-down direction, and, 50%+ perpendicular to the screen.
You can click inside the colored block to change the color of the incoming light.
Part of the incoming ray is reflected back to the atmosphere. (indicated by number 1)
The intensity of each polarized component is shown along the ray path.
Parts of the red light refracts into the drop, then refracts back to the atmosphere.( number 2)
Some reflects from the drop's inner surface, and refracts back to the atmosphere.(number 3) --> Rainbow.
Some reflects again, then refracts back to the atmosphere.(number 4)
You can drag the incoming ray, move it up and down, and watch how the relative intensity changes.
R is the radius of the water. b is the vertical distance from incoming ray to the center of the circle.
You can click inside the white box, and see what will happen.
When light refracts, it follow the law of refraction ni sin(£ci)=nr sin( £cr)
where n is the index of refraction.
Number in the left-bottom corner : i is the angle of incidence, r is the angle of refraction.
Most of the light are refract out from ray 2.
If your eyes intercept the separated colors from raindrops,
the red you see comes from drops angled slightly higher in the sky than dose the blue.
You can see a circular arc of color, with red on top and blue on bottom.
Clicks inside the colored box to show this effect. You can drap one of the rain drop, Try it!
Did yo notice that the rainbow consists of partially polarized light?
The intensity of the light coming from ray3/ray4 verse viewing angle are also provided.
There are two curves : ¡]Click ^ or v button to change its vertical scale)
| name | direction of electric field | color |
| s-wave | perpendicular to the screen ( + ) | same color as the light |
| p-wave | in the up-down direction ( | ) | yellow |
This is an e-mail message from Neal Rasmussen (posted with his permission)
I am a tornado chaser for 20 years. About 4 years ago I was driving through a McDonald's drive thru.
As it was just after a pretty good storm, I had mys polarized yellow tinted flip-up sunglasses on.
Low and behold I almost dropped my cigarette into my lap. There off in the east was a double rainbow!
Not the kind I've seen dozens of times where the second one is reveresed and much outside the primary,
but this was immediatly INSIDE the primary!
Gaping with my mouth open and people behind me honking,
I flipped my flip-ups up and the second inner one went away!
I must have sat there for 2 minutes flipping my sunglasses.
The colors were NOT reversed and it looked like one continuous double wide rainbow,
with two complete sets of colors.
I ruled out that the flip-ups where giving double vision as I looked at lights the next day,
streetlights headlights, etc. and no doubles!
Can this possibly be? Could a source of polarized light be at a different angle,
perhaps the sun shining through a cirrus cloud or noctelucent (sp).
Any suggestions! Please click hwang@phy03.phy.ntnu.edu.tw
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