Things are transparent that don't absorb, scatter or reflect the light. For stuff like quartz the band gap (energy above which photons get absorbed) is higher t

Things are transparent that don't absorb, scatter or reflect the light. For stuff like quartz the band gap (energy above which photons get absorbed) is higher than the energy of visible light so the light just goes through and its transparent... Are you asking why certain materials don't absorb visible light?