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  4. Red, blue, green and violet colour lights are one by one made incident on a photocathode. It is observed that only one colour light produces photo-electrons. That light is

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Q. Red, blue, green and violet colour lights are one by one made incident on a photocathode. It is observed that only one colour light produces photo-electrons. That light is

J & K CETJ & K CET 2015Dual Nature of Radiation and Matter

Solution:

Order of wavelength of given coloured lights are
$\lambda_{v}<\lambda_{b}<\lambda_{g}<\lambda_{r}$
Similarly, frequency order of these coloured lights are
$f_{v}>f_{b}>f_{g}>f_{r}$
Thus, energy order of these lights are
$E_{v}>E_{b}>E_{g}>E_{r}$
Therefore, violet light can produce photo electrons.