JEE Main · 2025 · 22 Jan 2025 · Shift 2 · Modern Physics

A light source of wavelength

λ

illuminates a metal surface and electrons are ejected with maximum kinetic energy of 2 eV . If the same surface is illuminated by a light source of wavelength

\frac{λ}{2}

, then the maximum kinetic energy of ejected electrons will be (The work function of metal is 1 eV )

(A)2 eV
(B)6 eV
(C)5 eV✓ Correct
(D)3 eV

Answer: (C)

Solution

By Einstein's photoelectric equation,

\frac{h c}{λ} = ϕ + K_{ma x}

.

For wavelength

λ

\frac{h c}{λ} = 1 + 2 = 3 eV

.

For wavelength

λ /2

, the photon energy doubles:

\frac{h c}{λ /2} = \frac{2 h c}{λ} = 2 (3) = 6 eV

So the new maximum kinetic energy is

K_{ma x}^{'} = 6 - ϕ = 6 - 1 = 5 eV

JEE Main · 2025 · 22 Jan 2025 · Shift 2 · Single correct option

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