Why is glass transparent?

Professor Phil Moriarty (University of Nottingham) discusses transparent glass and energy gap.

PS MESSAGE FROM PROF MORIARTY: "I misspoke for the last sentence of the video. What I meant to say is that the photons don't have enough energy to excite electrons across the energy gap and therefore light is transmitted. What I said was precisely the opposite, contradicting everything else in the video. Apologies for any confusion caused."

Source: Sixty Symbols