in the photoelectric effect, electrons are emitted from matter (typically metals and non-metallic solids) as a consequence of their absorption of energy from electromagnetic radiation of high frequency (short wavelength), such as ultraviolet light
When electromagnetic radiation interacts with an atom, it either excites electrons to a higher energy level known as an excited state, or, if the energy of the light is sufficiently high, it can ionize the atom by removing the electron
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This frequency is called the threshold frequency.