During 1886 and 1902,Hallwach and Lenard studied phenimenon of photoelectric emission .
Lenard.s experiment:
As shown in figure ,two metal electrodes are kept at opposite ends inside evacuated glass tube.
One electrode is called cathode (emitter) which is photo sensitive surface second electrode is called collector A.Potential difference is applied between C and A.
quartz window is provided in this tube through which ultraviolet rays are incident on cathode C.
When ultraviolet rays are incident on cathode (C) is stopped then current in circuit will become zero.
anode (A) is kept at positve voltage w.r.t. cathode (C ) hence it attracts electrons emitted and there is flow of electron.Hence,current flows in the circuit.
When ultraviolet radiation are stopped,current flowing through the circuit will stop
Lenard studied how photo current changes with collector plate potential with frequecny and intensity of incident light.
Hallwach.s experiment:
Hallwach in 1888,connected electroscope with negatively charged zinc plate as shown in figure
Following were his observations:
(i)When UV light was incident on zinc plate it lost its charge.
(ii)Iniially when UV light was incident on chargeless zinc plate,it became positively charged.
(iii)When initially zinc plate was kept positively charged and UV light was incident,positive charge on the plate increased.
From these observation he concluded when UV light is incident on zinc plate,negatively charged particles are emitted from the plat.
He also concluded that when frequency of incident radiation is less than some minimum frequency ,photoelectrons are not emitted.
