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.
ϕ0=hcλ−eV0=6.6×10−34×3×108640.2×10−9−1.6×10−19×0.54=3.093×10−19−0.864×10−19=2.229×10−19 J=2.229×10−191.6×10−19=1.39eV
Step 2: Find the new stopping potential.
The wavelength of the radiation emitted from an iron source, λ'=427.2nm=427.2×10−9 m
Let V0 be the new stopping potential. Hence, photo-energy is given as:
eV'0=hcλ'−ϕ0=6.6×10−34×3×108427.2×10−9−2.229×10−19=4.63×10−19−2.229×10−19=2.401×10−19 J=2.401×10−191.6×10−19=1.5eV
Hence, the new stopping potential is 1.50 eV.
© 2025 GoodEd Technologies Pvt. Ltd.