# Special relativity and rest mass of a photon

1. Mar 20, 2012

### Lizwi

1) How can get work done on a photon because work done = kinetic energy it depends on mass, but the rest mass of a photon is zero
2) From this equation E=PC , does this mean E of a photon is 0 because P must be zero since it depends on mass?

2. Mar 21, 2012

### clamtrox

Remember that classical mechanics does not work for things moving at the speed of light. You need to use special relativity instead. Just because something has zero mass doesn't mean that its momentum needs to be zero.

3. Mar 21, 2012

### granpa

4. Mar 21, 2012

### Mark M

Hi Liziwi,

The equation for finding the energy of a photon is
$$E= \frac{\hbar c}{\lambda }$$
Where E is energy, ${\hbar }$ is Planck's constant, $c$ is the speed of light, and ${\lambda }$ is the wavelength of the light.
Also, because E=mc2, an observer in relative motion with a photon will perceive it as having relative mass.

5. Mar 26, 2012

### ZikZak

Really? What experiment does he employ to perceive this "mass" of a photon?

6. Mar 26, 2012

### Mentz114

All observers are always in motion wrt to light with speed c. Photons are massless.