Final answer:
The retrograde light flow in an image intensifier tube would activate the photocathode, which is the component that ejects electrons in response to light based on the photoelectric effect. These electrons are then multiplied and eventually converted back into visible light by the output phosphor.
Step-by-step explanation:
If the present, retrograde light flow in an image intensifier tube will activate the photocathode, it is essential to understand the function of various components in devices like photomultiplier tubes and image intensifier systems. These systems work on the principle of the photoelectric effect, where light striking a metal plate (or photocathode) ejects electrons, which are multiplied in the subsequent stages, resulting in an amplified signal. This amplified signal can then be utilized for various sophisticated applications, such as in scintillators that detect particles. The output phosphor in these systems is responsible for converting the electrical signals back to visible light, whereas the focusing lens would be used to direct the light or electrons to the right place, and the input phosphor is what initially absorbs the incoming photons or particles and converts them into light.