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Traveling Photons
Name: Robert
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2000-2001
Question:
Please tell me if I am correct or not. A photon is not
always traveling at the speed of light because other scientists have
managed to demonstrate that they can slow them down. The photon is a
packet of light -- energy of the EM spectrum. If one slows down the
light, it thus loses its energy. Therefore, if light's (and the
photon's) energy can be changed, the mass is directly proportional to the
energy. If that was true, that be violating Dalton's Law of Conservation
of Mass, which says that matter cannot be created nor destroyed, because
energy has no mass. And if it were not violated, it would mean that a
photon is really a subatomic matter particle that hass mass and takes up
space.
Replies:
The light does not actually travel more slowly when in material. The light
travels from atom to atom. An atom absorbs the light for a short time and
then releases it. These short delays are what cause the apparent slowing.
When a lens is viewed as billions of atoms, light travels at the speed of
light as it moves from atom to atom. When viewed as one lens, the light
seems to just move at a slower speed.
As for Conservation of Mass, that is a theory that applies so long as
Einstein's E=mc^2 doesn't come into play. Mass appears to actually be a
form of stored energy. When the light is absorbed by an atom, the mass of
the atom slightly increases. When the light is released, the mass drops
back to the original value. We don't notice this because of the huge
proportion (the square of the speed of light). The small energy in a photon
of light is much too small to become a noticeable amount of mass. To get a
big change of mass, you need the energy of an atomic bomb. As long as no
nuclear reactions are happening, Dalton's law works.
Mellendorf
You are entertaining several misconceptions. First of all, the "speed of
light" that is often cited as a fundamental, invariant constant is actually
the speed of light IN A VACUUM. When light travels through a material, such
as glass, water, or even air, it moves more slowly. Its speed depends on
both the material and the frequency of light. The ratio of the speed of
light in a vacuum to the speed of light in the material is the "refractive
index" of the material.
When a photon of light travels through a medium, its energy is NOT changed,
unless the photon is actually absorbed by the medium. Its energy depends
solely on its frequency. Refraction of light does not change its energy.
There is no reason that a photon's mass should be directly proportional to
its energy. In fact, a photon has no mass. If it did, it could not travel
at the speed of light.
It also turns out not to be quite correct to say that "energy has no mass."
Under certain conditions, energy can be changed to mass and mass can be
changed to energy. The conversion factor is the famous E = mc^2. As far as
modern physics understands it, matter appears to be a sort of concentrated
form of energy.
If this doesn't clear things up for you, ask again and explain what doesn't
make sense to you.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Robert,
I think I understand what you are asking and will try to answer your
question.
Your statement that a photon has 'mass and takes up space' is not correct.
For matter the KINETIC energy depends on it's speed, but a photon is
not matter.
It is, as you said, a "packet" of light. Its energy depends on the FREQUENCY
(vibrations per second) of the oscillations of the electric/magnetic
fields in the
light/photon. The frequency does not change when the speed is slowed by
passing
through glass or other matter. Since the frequency doesn't change the
energy doesn't
change.
A couple of related comments:
A photon IS light and thus, by definition, is always travelling at the
speed of
light. What you referred to is the fact that the speed of light when
travelling
through matter (air, water, glass, etc.) is slower than the speed of light
IN VACUUM.
Since the frequency of light doesn't change when the speed slows the
WAVELENGTH
(the distance between consecutive peaks in the vibrations) does change --
it gets
shorter.
Greg Bradburn
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Update: June 2012
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