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 surfaces, then photo-electric effect is possible for the case of

Solution

Physics sir · 2 steps

Find the photon's energy in eV first, then check which work function it can beat.

1. The condition. Photoemission happens only if one photon carries at least the work function: E = hc/λ ≥ φ. Intensity does not matter, only the energy of each photon.

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

   Metal	Work function φ	Is 2.25 eV ≥ φ?	Photoemission
   Cs	1.9 eV	yes	happens
   Li	2.5 eV	no	does not happen

2. So. The 2.25 eV photon clears cesium's 1.9 eV but falls short of lithium's 2.5 eV. Only Cs emits electrons, which is option (d).

The idea to keep. Keep hc ≈ 1240 eV nm ready: divide by λ in nm and you get the photon energy in eV, which compares directly with any work function.

Answer from the NTA final answer key. Solution by PrepWiser.