Changing Photon Frequency
If color is a function of the light energy, is it possible
to pass, for example, green light through a medium of some kind
which would slow it down, i.e. absorb some energy, and thereby
transform the green light into, say, red light? In short, is it
possible to convert light of one color into light of another color?
The energy of a photon is not like the kinetic energy of a moving ball.
Kinetic energy of a ball or other macroscopic object is proportional to the
square of velocity, and you can change the velocity to any value you want
(until you get to the speed of light...). Light is different, though. While
the speed of light does change depending on the medium, the energy of a
photon is a function of its frequency, not its velocity. Photon energy is
quantized (e.g. it only exists in discrete 'packets'). You cannot change the
energy of a photon continuously -- you have to skip among specific levels.
Even if the photon 'slows down' slightly in a given medium, its frequency is
unchanged, and therefore so is its energy.
That does not mean you cannot change its wavelength, though. There are many
processes where light is absorbed by a material and ejected with a different
wavelength. One that might be familiar to you is fluorescence, where light
of higher energy /higher wavelength is absorbed (e.g. green), then emitted
at a lower wavelength (e.g. red). The photon is not really being 'slowed
down', but the wavelength in is different than the wavelength out.
I hope this helps!
This does not happen because of how a medium absorbs the energy. The
beam of light is made of a very large number of photons. Photons are
objects that are not exactly particles and not exactly waves. They are
little bundles of energy that travel together as a wave of light. When
something as small as a molecule interacts with them, things happen to
single photons. Each photon in the beam has the frequency of its color,
of its energy. If the light is purely one color, purely one frequency,
then all of the photons have the same frequency and energy. When energy
is absorbed by the medium, individual molecules absorb individual
photons. This makes the light dimmer, decreasing the total energy of
the entire beam, but does not change the energy or frequency of the
individual photons that remain.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012