[Physics] How is energy conserved during simultaneous refraction and reflection of light

energy-conservationintensityreflectionrefractionvisible-light

My question is regarding the partial reflection and refraction of light when incident on a refracting surface.

Here, the energy of Incident Ray is E=hv of Reflected Ray is E=hv and that of Refracted Ray is also E=hv.
I think so because neither refraction or reflection are accompanied by change in frequency(v) or so I have been told.

Doesn't this violate the pirinciple of conservation of energy?

As the initial energy is hv and final is 2hv.

something like this.

Best Answer

I will give you an answer on the QM level.

When a photon interacts with an atom, three things can happen:

  1. elastic scattering, the photon keeps its energy, and phase, and changes angle

  2. inelastic scattering, the photon gives part of its energy to the atom and changes angle

  3. absorption, the photon gives all its energy to the atom

In the case of reflection, it is 1. that is elastic scattering.

In the case of refraction, it can be 1. or 2. or even partially 3., that is, the photon can be elastically scattered (glass) or inelastically scattered (UV heating up the molecules), or absorbed. Now the photon can even be partially absorbed, that is, the photon can excite multiple atoms.

You are saying that energy is E=h*f for light. Let's take twp possibilities:

  1. you are talking about a single photon, in this case this photon will either reflect, or refract, it cannot do both at the same time (of course there is possibility for partial absorption)

  2. you are talking about a herd of photons, that we call classically light. IN this case you mean that the herd of photons has together E=hf. In this case, some of the photons will be reflected, and some will be refracted. Now in the case of glass, when you look at a glass window, it lets some of the light through, as a mirror image, that is refraction. But the glass window can reflect too, like a mirror. So the herd of photons has E=hf. Now some of the photons will be reflected, and some refracted. Let's say that the ratio is 50%. Now half of the photons together will have E/2=hf/2, and the other half will together have E/2=hf/2. Energy is conserved. This assumes, that the reflected and refracted photons will all keep their energy, which is not true, because in the case of refraction, part of the energy will be transformed into the atoms' energy.

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