This question came up in one of my conversations with a friend. What is the mass of light? Well, there are a couple of equations relating to energy and light. The most famous was developed by Albert Einstein: E = mc2, where E is energy, m is the rest mass, and c is the speed of light in a vacuum (3.0 x 108m/sec)
Another very famous equation is Planck's relation: E=hc/y, where E is energy, h is Planck's constant (6.626 x 10-34 J s), c is the speed of light in a vacuum (3.0 x 108 m/sec), and y is the wavelength of light.
So putting these equations together gives the following: E = mc2 = hc/y. Thus m = h/cy. The wavelength of visible light is from 400 to 700 nm. So to calculate the rest mass of light in the visible range, that has a wavelength of let's say 500 nm, we plug in the information and get the following:
m = 6.626 x 10-34 J s/ (3.0 x 108 m/s x 500 x 10-9 m)
m = 4.417 x 10-36 kg
So there you have it. The rest mass of a single photon of light at a wavelength of 500nm is 4.417 x 10-36 kg. Since light is never at rest, this number is actually the "effective" mass of a photon of light.
Another quick calculation is the momentum of a photon of light. Momentum is defined by the following equation: p = mv where p is the momentum, m is the mass, v is the velocity. Using the above mass for light at 500nm wavelength, and plugging the numbers into the momentum equation gives:
p = (4.417 x 10-36 kg) x (3.0 x 108 m/s)
p = 1.325 x 10-27 kg m/s
So the effective momentum of light is 1.325 x 10-27 kg m/s.
Source: http://open.salon.com/blog/kwatts59/2009/02/18/physics_what_is_the_mass_of_light
