The work functions of cesium (Cs) and lithium (Li) metals are 1.9eV & 2.5eV, respectively. If we incident a light of wavelength 550 nm on these two metal surfaces, then photo –electric effect is possible for the case of 1) Li Only 2) Neither Cs nor Li 3) Both Cs and Li 4) Cs only

Q. The work functions of cesium (Cs) and lithium (Li) metals are 1.9 eV and 2.5 eV, respectively. If light of wavelength 550 nm is incident on the surfaces of these two metals, the photoelectric effect will be observed in which of the following cases?

1. Lithium only
2. Neither cesium nor lithium
3. Both cesium and lithium
4. Cesium only

(JEE Main PYQ, Date: 22.01.2025, Shift: 01)

Ans: 4.Cesium only

Solution: The key condition for the photoelectric effect to occur is:

Photon energy (hν) ≥ work function (φ) (or equivalently, wavelength λ ≤ threshold wavelength λ₀)

First, calculate the energy of the incident photon (λ = 550 nm):

E = 1240 / λ (in nm) ≈ 1240 / 550 ≈ 2.25 eV

Now compare with the work functions:

• Cesium (Cs): φ = 1.9 eV → 2.25 eV > 1.9 eV → possible (photoelectrons are emitted)
• Lithium (Li): φ = 2.5 eV → 2.25 eV < 2.5 eV → not possible (no photoelectrons)

Therefore, the photoelectric effect occurs only for cesium.

Correct answer: 4) Cs only