# Photoelectric Solar Sail

1. Sep 19, 2009

### sanman

Suppose you have a solar sail that was capable of perfect photoelectric conversion.

So this means that photons from the Sun are striking your solar sail, and also being photoelectrically converted to electrical current.

My question is - does the photoelectric conversion at all affect the momentum transfer of the photons to the solar sail?

2. Sep 19, 2009

### Naty1

If conversion was PERFECT,all photon energy would be converted to electrical....this would include momentum....so it would appear some current momentum would be converted to electrical in a "PERFECT conversion.

via your established criteria, seems like it....whether that's practical or not is another question..such as solar cells getting warm in the sun....

Edit: Ignore the above...it seems the poster is addressing spacecraft solar SAILS....I wasn't paying attention....I missed photoELECTRIC instead of photoVOLTAIC....

Last edited: Sep 20, 2009
3. Sep 19, 2009

### sanman

So only SOME photon momentum would be converted into electrical energy, or ALL of it would?

4. Sep 20, 2009

Staff Emeritus
No, momentum is still conserved in an inelastic collisions.

Normally solar sails are designed to be reflective, so you have an elastic collision between light and the sail. To make a photovoltaic sail, it would have to be absorptive, so you have an inelastic collision, with only half the momentum transfer.

5. Sep 21, 2009

### sanman

Why half?

6. Sep 22, 2009

### PhaseShifter

If the photon is absorbed, its momentum changes from p to 0, so the momentum of the sail increases by p.

If the photon is reflected, the momentum changes from p to -p, so the momentum of the sail must increase by 2p. (actually it will be slightly different than this due to the fact that there is also energy transferred, resulting in a different wavelength.)