photo-electric effect

Electrons can be produced by ultra-violet light shining on the surface of a suitable metal plate . the rate of emission of electrons is low , so the current is very small , and the pulse electroscope can be used to measure it .
To demonstrate the emission of electrons by an illuminated zinc plate , (a) an ultra-violet lamp , such as Phillip’s TUV-6 watt , is required , (b) the zinc plate must first be prepared . For the preparation , a small globule of mercury is placed on the zinc plate and wiped over the plate with cotton wool moistened with dilute sulphuric acid . the plate is wiped dry with clean tissue and should be mirror-bright .

The zinc plate is clipped on to the support rod so that its bright surface faces the rod T . The ultra-violet lamp is then Placed beside the electroscope , so that it illuminates the zinc plate . The lamp must not be viewed when lit , as this is injurious to sight .
The high voltage is set at about +3kV and the gap to 1-2 mm . the flag then beats at a steady rate . The ultra-violet lamp is now switched on , and the flag is observed to beat at a different rate . when the high voltage is reduced to +1.5kV , the rate of beating is reduced . when the high voltage is changed to -1.5kV , no change occurs in the rate of beating , thus the carriers of the current between the zinc plate and T are negative particles . they are , in fact , electrons , emitted from the zinc .
When glass is inserted between the lamp and the zinc plate , no change is observed in the rate of beating for a high voltage of +3kV . thus glass does not transmit ultra-violet rays . when quartz is inserted the rate of beating alters , showing that quartz transmits ultra-violet rays .