JEE Main · 2025 · 22 Jan 2025 · Shift 1 · Modern Physics

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The work functions of cesium (Cs) and lithium (Li) metals are 1.9 eV and 2.5 eV, respectively. If we incident a light of wavelength 550 nm on these two metal surface, then photo-electric effect is possible for the case of

(A)Li only
(B)Cs only✓ Correct
(C)Neither Cs nor Li
(D)Both Cs and Li

Answer: (B)

Solution

The photon energy at 550 nm is

E = \frac{h c}{λ} = \frac{1240 eV\cdotnm}{550 nm} \approx 2.25 eV .

Photoelectric emission occurs only when the photon energy exceeds the metal's work function. Here

E = 2.25 eV

is greater than the work function of caesium (1.9 eV) but less than that of lithium (2.5 eV).

Photoelectric effect is possible for caesium only.

JEE Main · 2025 · 22 Jan 2025 · Shift 1 · Single correct option

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