The photoelectric effect is often cited as evidence for the particle nature of light and played a crucial role in the development of quantum mechanics. The phenomenon occurs when light (usually in the form of photons) interacts with a material, causing the ejection of electrons from its surface.
The key observations that the photoelectric effect explained were:
Threshold frequency: There is a minimum frequency of light below which no electrons are emitted, regardless of the intensity of the light. This threshold frequency is specific to the material and is independent of the light's intensity.
Instantaneous emission: Electrons are emitted almost instantaneously upon the absorption of a single photon. There is no time delay, even for low-intensity light.
Energy dependence: The maximum kinetic energy of the emitted electrons increases with the frequency (or energy) of the incident photons. Higher-frequency photons result in higher-energy electrons.
These observations cannot be fully explained by classical wave theory, which treats light as a continuous wave. In wave theory, the energy of a wave is expected to be proportional to its intensity (amplitude squared), rather than being related to the frequency. However, the photoelectric effect can be explained consistently by considering light as consisting of discrete packets of energy called photons.
According to the particle nature of light, each photon carries a specific amount of energy, which is determined by its frequency (or wavelength) according to the equation E = hf, where E is the energy of the photon, h is Planck's constant, and f is the frequency of the light. When a photon interacts with an electron in a material, it transfers its energy to the electron, which can then be ejected from the material if the energy exceeds the binding energy of the electron.
Regarding your second question, if we say that light is made up of photons, it does indeed imply that all electromagnetic radiations, including visible and non-visible radiation, are made up of photons. Photons are the quantum particles associated with electromagnetic radiation. The energy of a photon depends on its frequency (or wavelength), with higher-frequency photons carrying higher energy. This holds true for the entire electromagnetic spectrum, which includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. In each case, the radiation is composed of photons, but the properties of those photons, such as their energy and frequency, differ across the spectrum.