Please help me understand

Question

Can you explain or give an example of why the frequency of light does not change upon reflection

Answer 1

Today physicists accept dual wave-particle nature of light. To answer your question, we will treat light as an electromagnetic wave. When electromagnetic wave propagates, it encounters with a denser medium, say static dielectric surface. Each dielectric has a certain structure. The electromagnetic wave transmits some energy to the particles of the matter. As a result, wave creates forced oscillations in bound electrons of the atoms dipoles of dielectric. Dipole, as a reminder, the system of two-point particles with opposite electric charge. These dipoles create their own electric field which add to the incident field of light. The new dipole changing fields have the same frequency as the driving field of incident electromagnetic field, because the new field comes from the oscillating dipoles which are oscillating at the frequency of the incident field. We are talking about linear dielectrics (for them, by definition, dipoles have the frequency of incident field). The secondary waves of all dipoles coming back to the surface and emit reflected light. If we are considering the ideal matter, then all energy will be coming back in form of reflected rays. In reality not all rays will reflect. Some of the energy will be lost, especially in the form of heat.

Answer 2

One way that we can think about this is from the perspective of light as a particle. Einstein's equation relates the frequency of a photon to its energy:

E = hf

From this equation, we can interpret a change in frequency to relate to a change in energy. A change in energy from reflection would violate the first law of thermodynamics, which tells us (among other more subtle items) that energy can be neither created nor destroyed. This implies that a change in energy at the reflecting surface would impart a change in energy that is ultimately responsible for the change in observed frequency. Without an applied force to supply that energy to our system, this cannot occur.

I hope this helps you!

Cheers!

Answer 3

What I'm going to term, Static Reflection, cannot and will not change the frequency.

A rope tethered on both ends, if you make a wave (pinch, pull, release or slap it or something), then they'll propagate to their respective ends (and in an ideal system) 'perfectly' reflect and look 'opposite' to what they were before and propagate back towards their original source and keep bouncing back and forth infinitely if in a perfect chamber.

Light, being particle-like and wave-like, will do a similar thing when reflecting off of a static, non-opaque surface, will bounce off of other surfaces and not change its frequency. The most ideal example I can think of would be a mirror (not dirty or broken or impurities and such). Light rays of certain frequencies will collide with the mirror and bounce out and away at some angle (depending on the microsurface "elevations").

I hope this helps! Message me in the comments if you have any questions, comments, or concerns!