Question

Why is glass transparent to visible light but opaque to ultraviolet and infrared?

Answer 1

Transmission and reflection happen differently for every type of wave. So, the rules about which objects aretransparent and opaque also change.Glass is transparent to visible light but opaque to infrared. ... Sunscreen isopaque to ultraviolet light, but it'stransparent to visible light.

Assuming ( thanks LLlAMnYP for the translation) UL means the ultraviolet part of the spectrum and IL the infrared part, it is an experimental fact that glass is transparent for visible light which is in between . This can be completely described within the classical theory which defines the index of refraction for the degree of transparency.

What happens when matter is not transparent to the incident light, as for ultraviolet and infrared for glass , is the same as what happens with all non transparent materials. Light may be reflected, or absorbed in the first few atomic layers.

One has to go to the quantum mechanical frame to understand the complexity. The atoms, molecules and the lattices composed out of them respond to specific frequencies in various ways. For transparent materials, the photons making up the light do not interact, or interact coherently, with the lattice and molecular states, losing no energy. At most some may be reflected at the surface . It is a matter of construction of the lattice too ( think diamond and chunk of carbon) Opaque materials have centers which interact with the incoming photons, absorbing them and turning the energy to heat, or also reflecting them from the surface in a disorganized fashion.

When energies of photons are large, like x-rays most materials are transparent because the probability of the photons to interact is small and many go through unscathed.