## How do photons have momentum? [ENDORSED]

$\lambda=\frac{h}{p}$

Rory_Grant_3E
Posts: 13
Joined: Wed Sep 21, 2016 2:57 pm

### How do photons have momentum?

Since momentum is mass times velocity, how do photons have momentum if they have no mass?

Andreea Voro 3A
Posts: 13
Joined: Wed Sep 21, 2016 2:56 pm

### Re: How do photons have momentum?

Well photons don't have momentum because they don't have mass. Like you said, momentum is mass x velocity and since photons have no mass, they have no momentum.

Andrew Nguyen 3O
Posts: 20
Joined: Wed Sep 21, 2016 2:56 pm

### Re: How do photons have momentum?

Photons can however, with enough energy, eject electrons. These electrons have mass and velocity and therefore, have momentum. So even though photons themselves don't have any momentum, their energy can create momentum on electrons.

Brandon_Wu_2L
Posts: 13
Joined: Wed Sep 21, 2016 2:56 pm

### Re: How do photons have momentum?  [ENDORSED]

Einstein's equation E= mc^2 is really just a shortened version of the real equation: E²=(mc²)²+(pc)². E=mc^2 is used for bodies with mass but are not moving while the other equation is used for bodies that are moving such as a photon.
So when a photon is massless we can simplify the equation to E=pc.
Thus a photon can still have energy and momentum without needing to have mass.
If there was some value for m (mass of a photon), then we would find a photon could never reach the speed of light.
In fact, any matter with mass cannot attain the speed of light.

Flamingos 3L
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

### Re: How do photons have momentum?

Photons themselves don't have momentum due to no mass. If you want to determine the wavelength of a photon, the E= wavelength * frequency equation is what you need to use.