Glass!

Glass is opaque because the electrons are very tightly packed and photons of a visible light wavelength (400-700nm) don't carry enough energy to cause an electron to "jump" to a different energy level. An "Energy Gap" is the difference between certain energy levels. So because the photon doesn't have enough energy to affect the electron, it isn't absorbed and instead passes straight through the glass! Letting the "light" in! Harmful UV rays are kept out, don't worry. Adam King 21/03/12