Electron emission from a metallic surface by application of light is known as
- (a)Thermionic emission
- (b)Photoelectric emission
- (c)High field emission
- (d)Autoelectronic emission
Correct — B, Photoelectric emission. Electrons in a metal are held in by an energy barrier called the work function, and they can escape only if something supplies at least that much energy. When light does the supplying, the process is photoelectric emission. Einstein explained it in 1905 by treating light as quanta: one photon of energy h times the frequency is absorbed by one electron, and if that energy exceeds the work function the electron leaves with the remainder as kinetic energy. This is why emission depends on the frequency of the light and not on its brightness — below a threshold frequency no electron comes out however intense the beam, and above it the number of electrons rises with intensity while their maximum energy rises with frequency. Einstein received the Nobel Prize in 1921 for this work.
- (a)Thermionic emission — Here the energy is supplied as heat rather than light — a filament is raised to a high temperature until electrons boil off its surface. It is the mechanism of the cathode in an old vacuum tube or cathode-ray tube, not of a light-driven cell.
- (c)High field emission — In field emission a very strong electric field at the surface makes the barrier thin enough for electrons to tunnel through it. It is driven by voltage, not by illumination, and is used in field-emission microscopes and displays.
- (d)Autoelectronic emission — Autoelectronic emission is another name for the same field-emission process described in option (c), so it fails for the same reason. Both options describe an electric field pulling electrons out, and neither involves light.
There are four standard ways of getting electrons out of a metal, and they are classified by the source of the energy. Heat gives thermionic emission, light gives photoelectric emission, a strong electric field gives field or autoelectronic emission, and bombardment by fast particles gives secondary emission. In every case the electron must be given at least the work function of that metal, a quantity that differs from metal to metal — caesium's is low, which is why photocells often use it.
Read the stem for the agent supplying the energy and match it to the name. Here the agent is light, so the answer contains the prefix 'photo'. Note that two of the four options, high field emission and autoelectronic emission, are the same process under different names, which is a useful signal that neither can be the intended answer in a single-answer question. The photoelectric effect matters far beyond the exam: it is the reason a photodiode, a solar cell, a light meter and an automatic street light work, and historically it is one of the experiments that forced physics to accept that light is quantised.
- The work function is the minimum energy needed to free an electron from a metal surface.
- Photoelectric emission occurs only above a threshold frequency, however intense the light.
- Einstein's photoelectric equation states that the maximum kinetic energy equals the photon energy minus the work function.
- The number of emitted electrons depends on intensity; their maximum energy depends on frequency.
- Thermionic emission uses heat, field or autoelectronic emission uses a strong electric field.
- Expecting brighter light to eject electrons from any metal; below the threshold frequency none are emitted.
- Confusing thermionic emission, which needs heat, with photoelectric emission, which needs light.
- Treating high field emission and autoelectronic emission as two different processes; they are one.
NDA names the agent — light, heat or an electric field — and asks for the process, so learn the four kinds of emission by the energy source of each.
Which of the following pairs is/are correctly matched? Theory/Law — Associated Scientist 1. Continental Drift — Edwin Hubble 2. Expansion of Universe — Alfred Wegener 3. Photoelectric Effect — Albert Einstein
- (a) 2 and 3 only
- (b) 3 only
- (c) 2 only
- (d) 1 only
Answer(b) 3 only — the photoelectric effect belongs to Einstein, while the first two pairs have their scientists swapped.
Attaches the effect to the man who explained it, which is the other half of what examiners want on this topic alongside the mechanism itself.
Which of the following pairs of physical phenomenon and the discoverer is/are correctly matched? 1. James Chadwick : Photoelectric effect 2. Albert Einstein : Neutron 3. Marie Curie : Radium Select the correct answer using the code given below:
- (a) 1, 2 and 3
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 3 only
Answer(d) 3 only
NDA's own version of the same trap, deliberately crossing Einstein with Chadwick — worth seeing once so the pairing is never in doubt.
- practice — not a real PYQ
In the photoelectric effect, the maximum kinetic energy of the emitted electrons depends on
- (a)the frequency of the incident light
- (b)the intensity of the incident light
- (c)the area of the metal surface
- (d)the duration of the illumination
Answer(a) the frequency of the incident light — intensity fixes how many electrons come out, frequency fixes how energetic they are.
- practice — not a real PYQ
Emission of electrons from a heated filament is called
- (a)thermionic emission
- (b)photoelectric emission
- (c)field emission
- (d)secondary emission
Answer(a) thermionic emission — heat supplies the energy needed to overcome the work function, as in the cathode of a vacuum tube.