Final answer:
The photocathode layer emits electrons in a process called photoemission, which is a part of the photoelectric effect observed when photons with sufficient energy strike a material.
Step-by-step explanation:
The photocathode layer mentioned in the question is a component that, when stimulated, emits electrons. This phenomenon is scientifically known as photoemission. The photoelectric effect is a well-documented process in physics, where a metal surface irradiated with photons of sufficient energy leads to the ejection of electrons. These ejected electrons, often referred to as 'photoelectrons', are a key part of the workings in devices such as photocells found in automatic door openers. If light, such as monochromatic radiation, is shone on a photoelectrode (the target material), and the energy of the incoming photons is above a certain threshold, photoelectrons are emitted and a current is generated.