How does a ldr work?

An LDR (light-dependent resitor) is a particular type of semiconductor (A material with a conduction rating in between that of a conductor and an insulator i.e. electricity flows through a semiconductor slower than through a conductor, but quicker than through an insulator).

Consider an atom as made up of just a nucleus with bound electrons. These electrons require varying amounts of energy to 'escape' from the atom. The semiconductor, as all known matter, is made up of atoms.

Light can currently be explained as being made up of packets of waves called photons where the frequency of these waves determines how much energy they are associated with (Energy = h * frequency: where h is a constant).

Therefore, if light associated with enough energy stikes the semiconductor, then the electrons requiring the equivalent energy to break the bonds will escape.

Electricity is concerned with a flow of electrons, therefore electrons escaping means there is an increase in electrical current.

This increased electrical current is equivalent to a drop in resitance and thus fluctuation in particular frequencies of light affects the resistance of the semiconductor.

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