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Question:

Two radiations of photons energies 1eV and 2.5eV successively illuminate a photosensitive metallic surface of work function 0.5eV. The ratio of the maximum speeds of the emitted electrons is?

1:1

1:4

1:2

1:5

Solution:

We know that, vmax = √[2m(KEmax)] = √[2m(EPh − W)]
v₁/v₂ = √[(1 - 0.5)/(2.5 - 0.5)] = √(0.5/2) = √(1/4) = 1:2