Why is glass transparent?

— Manda Venkata Apparao, Hyderabad

Glass is transparent for two reasons.

First, light carries energy in small packets called photons. Electrons in atoms can absorb these photons and move from one energy level to another — but only if a photon supplies the right amount of energy.

Ordinary window glass is usually made of silicon dioxide, whose electrons need more energy to move into higher energy states than photons of visible light possess. As a result, most visible-light photons are not absorbed and pass through, making the glass transparent.

However, ultraviolet photons carry more energy and are absorbed by the electrons in glass. So glass is opaque to ultraviolet light.

Second, glass has a disordered atomic structure, meaning its atoms don’t have a regularly repeating arrangement. (This is unlike in a crystal.) However, this disorder occurs on a scale much smaller than the wavelengths of visible light. As a result, ordinary glass does not scatter much visible light, which travels through.

This is the same reason powdered glass appears white rather than transparent, as each fragment has surfaces that reflect and redirect light. With countless fragments randomly arranged, the light becomes scattered in many directions, preventing us from seeing clearly through the powder.