PHYSICS DOUBT--

Question

To every action there's an equal and opposite reaction. So can we say that light gets reflected due to this reason? That a ray gets reflected due to the reaction force? Also does a photon have mass? Because to exert some force on the object light should have some mass

Answer 1

Hello 0825 2.,

To really understand this you need to go deeper in physics, ultimately, understand Einstein's equation and classical electrodynamics (photons are electromagnetic waves, dual behavior).

Einstein's equation: E² = p²c² + m²c⁴

In the above equation, E is the total energy of the particle, p is the momentum of the particle (which is related to its motion), c is the speed of light, and m is the mass of the particle. This equation can be derived from the relativistic definitions of the energy and momentum of a particle.

The above equation tells us that the total energy of a particle is a combination of its mass energy and its momentum energy (which is not necessarily related to its mass). This where your confusion arises from. Relativity and particle physics is a whole another world of physics!

When a particle is at rest (p = 0), this general equation reduces down to the familiar E = mc². In contrast, for a particle with no mass (m = 0), the general equation reduces down to E = pc.

Since photons (particles of light) have no mass, they must obey E = pc and therefore get all of their energy from their momentum.

So, how can an object have momentum without mass? It can do this if it is a wave, and yes, photons flows are waves! A wave transports momentum via its waving motion and not by physically transporting an object with mass.

An example: If one girl shakes her end of the rope violently enough to send a wave down the rope to the other girl, the wave can jerk the other girl. The rope has not transported any mass, but it still carries momentum through its waving motion. In this way, waves can have no mass but still carry momentum. In addition to being a particle, light is also a wave. This allows it to carry momentum, and therefore energy, without having mass.

Hope that helps!

Answer 2

Hi 0825 2., (again!)

No, you don't need mass to impart an impulse (that's what photons do when they reflect). All you need is a momentum, which gets reversed in direction. Photons have momentum, but not mass. The equation is: p = h / λ . If you wanted to get micro-micro, the photon goes down and messes with the layer of surface electrons on the silvered (possibly, could also be aluminum) reflective layer. It digs in and gets kicked back, but that happens without physically touching them, it's all done with electrical fields. If you have something big, like your hand, technically it does the same thing when you swat the mirror, it's all electrons hitting other electrons! The difference is, you can start and stop your hand, and determine its inertial mass thereby. You can't do that with photons -- if you stop them, they disappear!

-- Cheers, --Mr. d.