The filter absorbs light of the unwanted wavelength. It has a pigment dissolved in the glass that absorbs light at a certain wavelength. The pigment is usually a transition metal oxide or a rare earth metal oxide. The elements have d-orbitals that can be promoted to a higher quantum number by photon collision. This energy will then be re-radiated in all directions thus reducing the amount in the forward direction and some is lost as heat. Another type of filter is a diachronic filter which uses a layer of metal (such as Aluminium) 1/4 wavelength thick that will totally reflect that wavelength. (I think it is metal but it could be a substance like fluorite, I am not sure). You see this effect when there is a thin film of oil on water. Opal also has a diachronic reflector that gives the bright colours. The transmitted light is white light minus the stopped light. If you stop blue light then the passed light will be green + red (=yellow). White light may be taken as a mixture or red + green + blue so stopping one or two will let the other(s) through. It ain't daemons. Chris.
