[Physics] Why does a mirror reflect visible light but not gamma rays

Question

Visible light (~500 THz) as well as gamma rays (~100 EHz) are electromagnetic radiation but we can reflect visible light using a glass mirror but not gamma rays. Why is that?

Answer 1

Look at the electromagnetic spectrum:

Visible frequencies have wavelengths of microns, $10^{-6}$ meters.

Gamma rays have a wavelength of $10^{-12}$ meters, picometers.

In physics, there are two mainframes, the classical frame, which includes Maxwell's electrodynamics, Newton's mechanics, and derivative theories, and the quantum mechanical frame which becomes necessary for small distances and high energies, where gammas (photons), electrons, atoms, nucleons, lattices belong.

The classical electromagnetic wave emerges from zillions of superposed photons. Maxwell's equations describe very well the behavior of light beams when scattering or reflecting or generally interacting for macroscopic distances and small energies. Reflection, classically, needs a very flat surface so that the phases of the reflected waves are retained. Depending on the material the classical beams may be absorbed, decohered in reflecting from many point sources, or reflected coherently if the scattering is elastic (mirrors elastically and coherently scatter incoming light).

Gamma rays though force us to go to the micro level, because of the very small wavelength that describes them as a light beam.

One has to look at the details of the surface, and whether a classical smooth surface for classical reflections can be modelled for gammas, and the answer is, no it cannot.

The spacing between atoms in most ordered solids is on the order of a few ångströms (a few tenths of a nanometer).

For micron wavelengths (optical light) the fields built up by atoms with angstrom distances in the lattice appear smooth and can be classically modelled.

Gamma rays considered as a classical light beam, with their picometer wavelengths see mostly empty space between the atoms of the solid.

An alternative analysis, still within the quantum frame, would be considering the photons which make up light, and the Heisenberg uncertainty $ΔpΔx$ in the location of the photon. For the small wavelengths of gamma rays, the photons see mostly empty space.