# E=mc2 e=hf

1. Mar 21, 2013

### huhjinsoo

e =mc2. e = hf

is it different????
than why??

..e =mc2 this only use in nuclear??

2. Mar 21, 2013

### HomogenousCow

E=mc^2 is the rest mass of a massive particle.
That other thing over there is the energy of a photon.

3. Mar 21, 2013

### Emilyjoint

If you put the two together and write f=c/λ then you can rearrange
mc2 = hc/λ to give λ = h/mc
DeBroglie did precisely this to come up with his hypothesis that λ = h/momentum
Which is the basis of wave - particle duality

4. Mar 21, 2013

### Drakkith

Staff Emeritus
The equation E=MC2 is used to find the energy content of some amount of mass. (Rest mass/invariant mass)

The equation E=hf is used to find the energy content a photon, where f = frequency of the photon.

Note that E=MC2 is actually part of a larger equation: E2=M2C4+P2C2, where P = momentum of the object. This allows you to find the energy content of an object based on its mass AND its motion relative to yourself.

5. Mar 22, 2013

### BruceW

as Emilyjoint is saying, if we take 'm' to be the 'relativistic mass', then e =mc2 and e = hf are both correct for a photon.

But as Drakkith is saying, if we take 'm' to be the 'rest mass', then the full equation is E2=M2C4+P2C2 and for a photon, the rest mass is zero, so E=PC for a photon.