photon theory of light?

Question

Certain types of black and white film are not sensative to red light. They can be developed with a red "safelight" on. Explain this on the basis of the photon theory of light ?

Answer 1

Light dependant reactions depend on electrons gaining energy. To gain energy they must absorb light, but electrons can only absorb light in packets ( photons). In quantum theory the energy of the photon is hf where h is planks constant and f is frequency( compared to amplitude squared in wave theory). The film isn't sensitive to red light because the electron needs more energy than the red light photon can provide! No matter how many packets you throw at it (brighter red light) it wont work! Look at http://en.wikipedia.org/wiki/Photoelectric_effect
which isn't directly related but the same principle.

Answer 2

Longer-wavelength photons are less energetic than shorter-wavelength ones. So red photons are the least energetic visible light photons. In the case of orthochromatic (red-insensitive) film, the red photons are below the minimum energy needed to expose a grain of the emulsion. If light were a continuous wave, you would expect that red light would expose the film too, it just might take a little longer.