Question 11.27: Monochromatic radiation of wavelength 640.2 nm (1 nm = 10-9 m) from a neon lamp irradiates photosensitive material made of cesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage.

Hint: Use Einstein's photoelectric equation.
Step 1: Find the work function.

ϕ0=hcλeV0=6.6×1034×3×108640.2×1091.6×1019×0.54=3.093×10190.864×1019=2.229×1019 J=2.229×10191.6×1019=1.39eV

Step 2: Find the new stopping potential.
The wavelength of the radiation emitted from an iron source, λ=427.2nm=427.2×109 m

Let V0 be the new stopping potential. Hence, photo-energy is given as:

eV0=hcλϕ0=6.6×1034×3×108427.2×1092.229×1019=4.63×10192.229×1019=2.401×1019 J=2.401×10191.6×1019=1.5eV

Hence, the new stopping potential is 1.50 eV.