Does light push back against the flashlight, in accordance with Old Newton's law?

Question

For example, if I turn on a flashlight, does the light push back against the flashlight? And how much light would have to be released in order for me to feel the push of the photons?

Answer 1

Have you ever seen a solar motor? They are in an evacuated bulb and contain four paddles mounted on a low friction bearing. One side of each paddle is silver and the other blackened. When light shines on the motor the paddles spin due to absorption of light photons and then re-radiation of IR photons.
Totally useless but it does demonstrate that the third law applies in this case.

Answer 2

Um... this question is indeed relativistic, but hardly incomplete.

Photons carry energy of E = pc

Rearranging this you can say p = hbar/lambda

So, yes, it is based on wavelength. Taking an average over white light will suffice though.

Since you carry the momentum away from the system, there must be a corresponding momentum change in the opposite direction. (It's somewhat akin to a one-dimensional explosion). Why can't you feel it? Check the value of hbar and lambda here.

hbar = 1.054*10^-34
lambda ~= 500*10^-9

hbar/lambda = 2.109*10^-28

Yup. That's small exponent. You can see that you'll need a lot of photons to give you any kind of kick whatsoever.

Answer 3

Yes it does. The force in newtons=P/c where P= power in watts of the light beam and c= speed of light, 300 000 000 metres per sec. So for a force of 1 newton, which is like a weight of 100 grams, you'd need a 300 megawatt light beam, which is the total output of a medium-size power station.

Answer 4

Your question provokes relativistic discussion and as such, it is incomplete and has too many answers to provide accuracy. The key to answering this question is the still open question of whether a photon has any mass. "Relativistic mass of a photon" would clearly depend on its frequency. You "feeling" the push of the photons would clearly depend on how sensitive the area that the flashlight is pushing against is. For example, if the flashlight is leaning against a calloused corn on your right foot, the rate of photonic emission would have to be significantly larger than if you had the flashlight firmly wedged within your rectum.

I rest my case. Good luck in your search for truth... and watch out for the slide switch!

Answer 5

Yes, as a matter of fact, it does. However, the effective mass of photons is extremely small. They are defined as having NO rest mass. However, lasers have been used to propel highly polished objects into the air ( I forget the website for the guy who did it, but it was quite interesting ). So with a sufficiently large bank of powerful lasers, you could propel a ship in space. On the ground, the friction from even the best bearing would be too much for such a drive to overcome.

Answer 6

zee prime said "you'd need a 300 megawatt light beam, which is the total output of a medium-size power station."

Dude. What would THAT do to the MOON?

Answer 7

I did the math once and if I remember correctly, a flashlight placed in orbit, pointing in one direction would increase its speed by about 1.5 meters/second by the time the batteries burnt out.

Answer 8

Yes. They have plans for space crafts with solar sails that will be pushed by suns ray. May be if you're floating out in space you might start to move slightly.