as thermionic emission . Metal Electrons liberated Termal energy (b) Electrons (a) Figure . Electrons in the (a) metal (b) heated metal The intensity of the thermionic emission (the number of electrons emitted) depends on the metal used and its temperature. Examples: cathode ray tubes, electron microscopes, x-ray tubes etc (Figure .
). Table . Work function of some materials Metal Symbol Work function (eV) Metal Symbol Work function (eV) Cesium Cs . Aluminium Al .
Potassium K . Mercury Hg . Sodium Na . Copper Cu .
Calcium Ca . Silver Ag . Molybdenum Mo . Nickel Ni .
Lead Pb . Platinum Pt . - - - - Unit dual nature of radiation and matter . PHOTO ELECTRIC EFFECT .
. Hertz, Hallwachs and Lenard’s observation Hertz observation Maxwell’s theory of electromagnetism predicted the existence of electromagnetic waves and concluded that light itself is just an electromagnetic wave. Then the experimentalists tried to generate and detect electromagnetic waves through various experiments. In , Heinrich Hertz was successful in generating and detecting electromagnetic wave with his high voltage induction coil causing a spark discharge between two metallic spheres (we have learnt this in Unit of XII standard physics).
When a spark is formed, the charges will oscillate back and forth rapidly and the electromagnetic waves are produced. The electromagnetic waves thus produced were detected by a detector that has a copper wire bent in the shape of a circle. Although the detection of waves is successful, there is a problem in observing the tiny spark produced in the detector. In order to improve the visibility of the spark, Hertz made many attempts and finally noticed an important thing that small detector spark became more vigorous when it was exposed to ultraviolet light.
The reason for this behaviour of the spark was not known at that time. Later it was found that it is due to the photoelectric emission. Whenever ultraviolet light is incident on the metallic sphere, the electrons on the outer surface are emitted which caused the spark to be more vigorous. iii)