Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: light losing energy

Learn more physics!

Most recent answer: 06/13/2017
Q:
Regarding light's mass/momentum. If light is able to push objects (space sails) by bouncing off (Newton's third law), how does it not lose energy? If the speed of light is constant, and energy is never created or destroyed, how does light push objects?
- Aidan
A:

When light bounces off an object, the object either gains or loses energy, depending on whether the push goes in the direction the object was moving or the opposite direction.The light then loses or gains exactly the same amount, keeping the total energy constant. What happens, in the typical simple case, is that the reflected light has a slightly different frequency than the original light. Even if every photon is reflected, each one will have then a slightly different energy than it had before.

That frequency shift that depends on the motion of the reflector (in our reference frame) is the key to how speed-measuring radar works.

Mike W.


(published on 06/13/2017)

Follow-up on this answer.