Energy Levels in Atoms and Line Spectra problem 5

In a solid, electrons in neighbouring atoms are close together and interact with each other. This interaction changes the energy levels of electrons. The presence of many atoms in a lattice causes a spread of energy levels into a band. This is why there are energy bands, rather than discrete energy levels, in a solid.

Photons of light incident on the light-dependent resistor (LDR) give energy to the electrons in its valence band; these electrons, therefore, become excited and move into the conduction band, leaving holes in the valence band. These electrons and holes are charge carriers, which therefore increase the current in the LDR, hence a decrease in the resistance of the LDR.

So the higher the light intensity, the more the photons, so more electrons in the conduction band. This, therefore, leads to more electrons and holes, therefore higher current in the LDR and hence, lower resistance.