effect The ejection of electrons from a metal plate when illuminated by light or any other electromagnetic radiation It is interesting to note that the experiment of Hertz confirmed that light is an electromagnetic wave. But the same experiment also produced the first evidence for particle nature of light. Hallwachs’ observation In , Wilhelm Hallwachs, a German physicist, confirmed that the strange behaviour of the spark is due to the action of ultraviolet light with his simple experiment. A clean circular plate of zinc is mounted on an insulating stand and is attached to a gold leaf electroscope by a wire.
When the uncharged zinc plate is irradiated by ultraviolet light from an arc lamp, it becomes positively charged and the leaves will open as shown in Figure . (a). Further, if the negatively charged zinc plate is exposed to ultraviolet light, the leaves will come closer as the charges leaked away quickly (Figure . (b)).
If the plate is positively charged, it becomes more positive upon UV rays irradiation and the leaves open further (Figure . (c)). From these observations, it was concluded that negatively charged electrons were emitted from the zinc plate under the action of ultraviolet light. + + + + + Zn –– –– – – – – – – – Zn – ––––– – ––––– ++ ++ + + + + + Zn (a) (b) (c) UV rays + ++++ Figure .
Irradiation of ultraviolet light on (a) uncharged zinc plate (b) negatively charged plate (c) positively charged plate - - - - Unit dual nature of radiation and matter of suitable wavelength (or frequency) is called photoelectric effect . Although these electrons are not different from all other electrons, it is customary to call them as photoelectrons and the corresponding current as photoelectric current or photo current . Metals like cadmium, zinc, magnesium etc show photoelectric emission with ultraviolet light while some alkali metals lithium, sodium, caesium respond well even to larger wavelength radiation like visible light. The materials which eject photoelectrons upon irradiation of electromagnetic wave of suitable wavelength are