Electrocyclic reaction
Light can move an electron up to an excited state that occupies a higher orbital. The excited electron will occupy the LUMO, which has a higher energy level than the electron's old orbital. If light opens the ring of 3,4-dimethylcyclobutene, the resulting electrocyclization would be occur by a disrotatory mode instead of a conrotatory mode. The correlation diagram for the allowed excited state ring opening reaction shows why: