Photo electric emission When an electromagnetic radiation of suitable frequency is incident on the surface of the metal, the energy is transferred from the radiation to the free electrons. Hence, the free electrons get sufficient energy to cross the surface barrier and the photo electric emission takes place (Figure . ). The number of electrons emitted depends on the intensity of the incident radiation.
Examples: Photo diodes, photo electric cells etc. Free electrons Radiation Emitted electrons Metal Figure . Photo electric emission iv) Secondary emission When a beam of fast moving electrons strikes the surface of the metal, the kinetic energy of the striking electrons is transferred to the free electrons on the metal surface. Thus the free electrons get sufficient kinetic energy so that the secondary emission of electron occurs (Figure .
). Examples: Image intensifiers, photo multiplier tubes etc. Free electrons Metal High Speed electrons Secondary electrons p d e Figure . Secondary emission of electrons - - - - Unit dual nature of radiation and matter Lenard’s observation In , Lenard studied this electron emission phenomenon in detail.
His simple experimental setup is shown in Figure . . The apparatus consists of two metallic plates A and C placed in an evacuated quartz bulb. The galvanometer G and battery B are connected in the circuit.
_ Ultraviolet Radiation G C Electrons Quartz bulb Figure . Experimental setup of Lenard When ultraviolet light is incident on the negative plate C , an electric current flows in the circuit that is indicated by the deflection in the galvanometer. On other hand, if the positive plate is irradiated by the ultraviolet light, no current is observed in the circuit. From these observations, it is concluded that when ultraviolet light falls on the negative plate, electrons are ejected from it which are attracted by the positive plate A .
On reaching the positive plate through the evacuated bulb, the circuit is completed and the current flows in it. Thus, the ultraviolet light falling on the negative plate causes the electron emission from the surface of the plate. Photoelectric