Question

Use the band structure of crystalline solids to
explain why a thin piece of sulfur, a good insulator,
transmits light that is reddish yellow when illuminated
from the back side with white light.

Answer 1

The band structure of crystalline solids is explained with the help of Kronig-Penney model.

This model says that there are allowed and forbidden values of energy for electrons in an atom.

The electrons can stay with only specified values of energy.

These values of energy that are allowed are called bands.

Between the allowed bands, there are forbidden bands.

For metals, the bands are half filled so that the rest of the band is empty and so, electrons can move freely with a small force.

In semiconductors and insulators, the bands are completely filled.

So, in order for the electrons to move, they have to move to the next allowed band, crossing a forbidden band.

This is why electric conduction is difficult in insulators and semiconductors.

Allowed region band gap Forbidden region O O O OOOOoº Allowed region electrons

For electrons to cross the band gap, an energy equal to or greater than the band gap energy of the material has to be supplied.

A light of appropriate frequency will have this energy.

This energy for sulfur comes in the range of the yellow-red light that is present in white light.

The electrons absorb the energy from white light and jumps to the higher energy level.

These electrons are highly unstable and comes back to its original position within a time period.

While coming back, the material emits the light that has the exact energy of the band gap.

Thai is why sulfur transmits reddish yellow light.