The only real problem in photography be it stock, microstock, art or otherwise, is and always has been light.

Light is the electromagnetic radiation visible to the human eye. In classical physics the wave of electromagnetic radiation has amplitude, which is the brightness of the light, wavelength, which is the color of the light, and an angle at which it is vibrating, called polarization (reflecting windows anyone?). In quantum physics electromagnetic radiation consists of particles called photons, which are packets ("quanta") of energy which move at the speed of light. The brightness of the light is the number of photons, the color of the light is the energy contained in each photon, and four numbers (X, Y, Z and T) are the polarization.

And as weird...