From: pauld@exploratorium.edu
Date: Sun Mar 03 2002 - 22:34:02 PST
Message-Id: <200203040633.g246Xwf23703@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole rainbows/prisms Date: Sun, 3 Mar 2002 22:33:58 US/Pacific
Hi Regan
The outer electron cloud around a nucleus in any atom (or nuclei in molecules)
will shake back and forth at a resonant frequency if the electron is pulled to
the side and released. In most atoms this electron cloud has a resonant frequency
in the ultra violet. Electromagnetic waves shake the outer electron cloud of an
atom back and forth at the frequency of the wave.
If you shine light onto atoms at this resonant frequency the light will be
absorbed or scattered. (If it is scattered the phase of the outgoing wave will be
delayed behind the phase of the incoming wave.)
Away from resonance i.e. in the blue the light is scattered less than the UV and
the phase delay is less.
Further away still in the red the light is scattered even less and the phase
delay is less.
So every atom or molecule delays the wave crests of an incoming light wave. The
result is that the energy is slowly removed from the original beam of light and
reradiated, the beam appears to slow down as it travels through the medium. Paul
Hewitt's model is that each atom absorbs a photon delays it for a while then
spits it out again. My model tells you why the delay for each different color or
frequency is different.
Blue is scattered more than red so the sky is blue.
Blue is delayed more than red so blue light bends more when it goes through a
prism than red light.
The sky is a prism and breaks the image of the setting sun up into a spectrum of
colors producing the green, blue or violet flashes seen at sunset.
Consider a swing. A student sits in the swing to act as a mass.
A teacher climbs to the top of the swing balances on the top bar reaches down and
starts pushing on the ropes of the swing.
Push at resonance and the motion of the student is large (your energy
representing the electromagnetic wave has been absorbed by the swing oscillation
the electron cloud around the atom.)
Push slower than resonance and less energy is absorbed. The motion of the swing
is out of phase with your pushes. Push even slower and the pendulum swings back
and forth less and the phase delay is less. I hear the phase pendulum exhibit is
back at the explo now, I'll go check this tomorrow. You can build your own
version of the phase pendulum and see the effect yourself.
* OK some substances have resonances in the infra red, like iodine. Make a prism
from iodine and the red will bend more than the blue. Make an atmosphere of
iodine gas and the sky will be red while the setting sun will be blue.
Paul D
> Hello.
>
> All explanations of rainbows/primsm say that the differnt wavelengths of light bend
differently. Why do they bend differently? My students are curiuos about this
and so am
I. I have looked online and in books but have not found out more than that
different
wavelengths travel at different speeds (is this true for any medium?).
> Also, is the explanation of the blue sky (scattering of violet and blue light) related
to
the explanation of how a prism works? If the atmosphere were super-thich (and
prism
shaped) could it make rainbows?
>
> Thanks.
> -Regan Brooks
>
>
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