Electrons are accelerated between a filament and a grid through mercury vapour by a variable potential V, as shown in figure 1. A small retarding potential, V R = 0.5 V, is maintained between grid and the collector plate. When the current I in the collector is measured as a function of the accelerating voltage, the curve of figure 2 is obtained.
(i) Which of the following is correct –
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Ans.
(i)
Sol. In order to reach the collector the electrons must have a kinetic energy greater than the retarding potential energy of about 0.5 eV between the grid and the collector.
(ii)
Sol. The voltage difference between the various current peaks corresponds to the energy required to excite mercury into its first excited state, so that
Δ E = e Δ V = 4.88 eV
(iii)
Sol. The wavelengths of the photon emitted when the excited mercury atoms return to their ground state is
λ = 
m λ = 2540 Å
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