Photo-tubes cut off (don't respond to photons) at sufficiently small wavelengths. This is because the photo-tube's glass does not transmit UV. They also cut off at sufficiently large wavelengths, where the glass does transmit the light. Why?
Photo-tubes operate on the principle of photoelectric effect. This effect is showing the particle nature of the light. When light of sufficiently small wavelength incident on the metal plate, the emission of electrons from the surface of ...
The conceptual question about photoelectric effect is answered and explained.