Sat October 27, 2012 By: Abhi Patel

Can light apply force? (F=ma) Please explain.

Expert Reply
Sat October 27, 2012
Newton's law F=ma applies for object of significant size (larger than a few molecules) and less than maybe one-tenth of the speed of light. It is an approximation that works very well in non-extreme situations. Light is not a particle in the strictest sense. It behaves as a bundle of waves. Its energy works out to be proportional to its frequency. Its momentum works out to be proportional to its wavelength. Light only exerts enough force to affect individual particles.

The best way to describe force of light on a particle is absorption and then emission of the light. When the light is absorbed, so are the light's energy and momentum. When the light is emitted, so are the energy and momentum. If the light is emitted in a direction different from the original direction, there is a change of momentum in the particle. This momentum change is seen as a change of the particle's motion.
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