A purely intuitive explanation:(posted 9331 days ago)Light is composed of little packets of energy called photons. The shorter the wavelength the more energy in each photon. Blue and UV photons have shorter wavelengths, and thus more energy, than green or red photons. When photons strike a silver halide crystal the structure is upset. Electrons are knocked out of orbits. It takes a minimum amount of energy to knock these electrons out of orbit. Unless the silver halide crystal is modified in some way only blue and ultraviolet photons have enough energy to do this.