LDR theory.
Bohr model → valence band → conduction band → band gap.
The Bohr model introduces atomic energy levels. In a solid, many closely spaced levels form bands. The band gap is the energy gap between them.
Red Bull gives you wings…
Two riverbanks, with a canyon between them.
You are an electron on one side. You want to reach the other.
In our story, your starting level is 5 and the other side is 7.4.
You need an extra 2.4 “Red Bulls” of energy to jump.
A photon with enough energy arrives and gives you that boost.
You “grow wings,” cross the gap, and can help carry current.
Less light → fewer energy boosts → fewer free charge carriers.
More light → more can “fly” → lower resistance.
The Red Bulls and the levels 5 and 7.4 are a story. In the material, the gap is about 2.4 electron volts, and a photon must supply enough energy for the transition.
The zig-zag: cadmium sulfide
Many familiar LDRs use cadmium sulfide (CdS), a semiconductor compound. The patterned surface forms a light-sensitive resistive path.
The energy gap
An electron can gain energy to move from the valence band into the conduction band, where it can contribute to current. The separation is the band gap.
A response to visible light
CdS has a band gap of about 2.4 eV. Higher-energy visible photons can bridge that gap.
The quantum hit
An absorbed photon can provide the energy for the jump. More light creates more mobile charge carriers, increasing conductivity and lowering resistance.
Visible photons have different energies. This is an energy threshold, not an exact match shared by every color. Energy-band reference ↗