# Proerties of light

1. Nov 23, 2013

### tdev

can light move matter?

2. Nov 23, 2013

### MuIotaTau

Light possesses momentum, so in that sense it can certainly move matter in the form of radiation pressure. Was there anything in particular you were looking for?

3. Nov 23, 2013

### tdev

But it doesnt have any mass to posses momentum,then how can it iteract with matter?

4. Nov 23, 2013

### MuIotaTau

In non-relativistic mechanics, we learn that momentum is given by $p = mv$, but in relativity it comes out that the relationship is actually $$E^2 = (pc)^2 + (mc^2)^2$$ where $c$ is the speed of light and $E$ is the energy. For a photon, $m = 0$, which gives us $$E = pc$$ And we know that photons have energy, so they must necessarily have momentum.

Additionally, as you would learn in introductory quantum mechanics, the de Broglie relationships state that the momentum of a photon is given by $$p = \frac{h}{\lambda}$$ where $\lambda$ is the photon's wavelength and $h$ is called the Plank constant. (This relationship actually turns out to be true for all particles, but that's another story.)

Note that I'm simply stating facts here, I haven't actually explained anything properly. There's a lot of physics that really contextualizes these relationships, but the basic point is that the notion of momentum just being the product of mass and velocity isn't really satisfactory.

Last edited: Nov 23, 2013
5. Nov 23, 2013

### Staff: Mentor

Also, even in classical electrodynamics, in which concept of "photon" does not appear at all, electromagnetic waves carry energy and momentum which are related by (surprise!) E = pc.

6. Nov 23, 2013

### sophiecentaur

Yes. If you do all the classical EM calculations for what happens when a beam of light falls on a reflecting surface, there is a resulting force on that surface which corresponds exactly to the rate of change of momentum of the equivalent rate of photons bouncing off it. Things like that are really refreshing, imo.