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Question:

The work function (Φ) of some metals is listed below. The number of metals which will show the photoelectric effect when the light of 300 nm wavelength falls on the metal is:
Metal | Li | Na | K | Mg | Cu | Ag | Fe | Pt | W
Φ(eV) | 2.4 | 2.3 | 2.2 | 3.7 | 4.8 | 4.3 | 4.7 | 6.3 | 4.75

1

2

4

3

Solution:

E = hc/λ = 6.6 × 10⁻³⁴ × 3 × 10⁸ / 300 × 10⁻⁹ J = 6.6 × 10⁻¹⁹ J = 4.125 eV
The metals having work function less than 4.125 eV will show photoelectric effect. Hence, Li, Na and K will show photoelectric effect. Therefore, the answer is 3.