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Name:  Robert
Status:  student
Age:  16
Location: N/A
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Date: 2000-2001

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.

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.


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


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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