Physics · Dual Nature Of Radiation And Matter · NEET
A metal has free electrons, but they are held inside by the positive ions. If one electron tries to leave, the surface becomes slightly positive and pulls it back. So the electron needs extra energy to break free. The minimum energy needed is called the work function (phi-0). Any of the three emission types simply supplies this energy.
In thermionic emission the extra energy comes from HEAT: the metal is heated so electrons gain thermal energy and boil off the surface (used in old vacuum tubes and CRTs). In photoelectric emission the energy comes from LIGHT: a photon of frequency higher than the threshold frequency hits an electron and knocks it out. Heat vs light is the key difference.
Field emission happens when a very strong electric field, of the order of 10^8 V/m, is applied to the metal. The strong field literally pulls electrons out of the surface. A common NCERT example is a spark plug, where the sharp point creates a huge field. No heating or light is needed here.
They point to the same event. Photoelectric emission is one of the three ways to release electrons (energy source = light). When we study this release in detail with graphs and Einstein's equation, we call the whole phenomenon the photoelectric effect. So photoelectric emission is the emission itself, and photoelectric effect is the broader topic built around it.
Yes. The work function phi-0 is a property of the metal, not of the method. Whether you heat the metal, apply a strong field, or shine light, the electron leaves only when it receives energy at least equal to phi-0. The three types differ only in HOW that energy is delivered, not in the amount needed.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Thermionic emission (energy from heat), field emission (energy from a strong electric field of about 10^8 V/m), and photoelectric emission (energy from light of suitable frequency).
Field emission. A very strong electric field near the sharp electrode pulls electrons out of the metal.
Heat. When the metal is heated to a high temperature, free electrons gain enough thermal energy to overcome the work function and escape.
The frequency of the incident light. Only if the frequency is above the threshold frequency (so that photon energy hv is greater than the work function phi-0) will electrons be emitted, no matter how intense the light is.
Yes. The work function is a fixed property of the metal surface. All three methods must supply energy at least equal to it; they only differ in the source of that energy.
