Final answer:
The photoelectric effect involves the emission of electrons from a metal when it is exposed to light of a certain threshold frequency or higher, where the emitted electrons' kinetic energy depends on the light's frequency.
Step-by-step explanation:
When studying the photoelectric effect, scientists observed that electrons were emitted from metal in response to a beam of light. This phenomenon occurs when light shone onto a metal surface causes the ejection of electrons from that metal. Only certain frequencies of light are able to cause the ejection of electrons. If the frequency of the incident light is too low, no electrons are ejected, regardless of the intensity. When the frequency of the light is above a certain threshold, known as the threshold frequency, then electrons are ejected, and the number of emitted electrons is proportional to the intensity of the light. Furthermore, the kinetic energy of the emitted electrons is proportional to the light's frequency, not its intensity. This discovery was crucial in the development of quantum mechanics and demonstrated that light can behave both as a wave and as a particle.