Sixty Symbols regular Professor Phil Moriarty discusses transparent glass and the so-called 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."

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(From Youtube)