Physics · Dual Nature Of Radiation And Matter · NEET
It emits electrons, not light. Light comes IN, and electrons come OUT. The metal absorbs the incoming light and uses that energy to eject electrons from its surface. These ejected electrons are called photoelectrons. The word 'photoelectric' means 'light causing electricity' — light frees the electrons, and moving electrons make a current.
Because emission depends on the FREQUENCY of light, not the brightness (intensity). Each photon must carry enough energy (E = h times frequency) to free one electron. If the frequency is below the threshold frequency, no single photon has enough energy, so no electrons come out no matter how bright the light is. Brightness only adds more photons of the same low energy — many weak photons still cannot free one electron. This is why UV light works on zinc but bright red light does not.
Light with a frequency at or above the threshold frequency of that metal. High-frequency light such as ultraviolet (UV) works for many metals. Metals like zinc, cadmium, and magnesium respond only to UV light (short wavelength, high frequency). Alkali metals like caesium, potassium, and sodium have low work functions, so even visible light can eject electrons from them.
Both release electrons from a metal, but the energy source is different. In thermionic emission you HEAT the metal, so heat energy frees the electrons. In the photoelectric effect you shine LIGHT of suitable frequency on the metal, so light energy frees the electrons. Same result (electrons escape), different cause (heat vs light).
Heinrich Hertz discovered it in 1887, by accident, during his experiments on electromagnetic waves. He noticed that sparks jumped more easily when ultraviolet light fell on the metal plate. Later, Hallwachs and Lenard studied it in detail (1886–1902) and found the idea of a threshold frequency — a minimum frequency below which no electrons are emitted.
The maximum kinetic energy of the emitted photoelectrons in the photoelectric effect is independent of:
The work functions of Caesium (Cs), Potassium (K) and Sodium (Na) are 2.14 eV, 2.30 eV and 2.75 eV respectively. If incident electromagnetic radiation has an energy of 2.20 eV, which of these photosensitive surfaces may emit photoelectrons?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the ejection of electrons from a metal surface when light of a high enough frequency falls on it.
Photoelectrons are the electrons that are knocked out of a metal surface by the incoming light during the photoelectric effect.
Yes. As soon as suitable light hits the metal, electrons are emitted with no measurable time delay. This instant response is one of the key facts that the old wave theory of light could not explain.
It is the minimum frequency of light needed to eject electrons from a given metal. Below this frequency, no electrons come out even if the light is very bright. Above it, emission begins.
It proved that light behaves as particles called photons, each carrying a fixed packet of energy. This is a core piece of the dual nature of radiation and matter, and Einstein explained it, winning the Nobel Prize for it.
