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NCERT Class XII Chapter
Dual Nature of Radiation and Matter
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Question : 27
Total: 37
Monochromatic radiation of wavelength 640.2 nm (1 nm = 10–9 m) from a neon lamp irradiates photosensitive material made of caesium 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 photocell. Predict the new stopping voltage.
Solution:  
From the first data, work function of given photosensitive material can be calculated.
E = hv = W0+
1
2
mvmax2
.63×10−34×3×108
640.2×10−9×1.6×10−19
 = W0+0.54 eV
1.94 eV = W0 + 0.54 eV or W0 = 1.4 eV
Now the source is replaced by iron source which produce 427.2 nm wavelength.
E = W0+
1
2
mvmax2
.63×10−34×3×108
427.2×10−9×1.6×10−19
 = 1.4 eV + (K.E)max
2.9 eV = 1.4 eV + (K.E)max or (K.E)max = 1.5 eV
Stopping potential required is 1.5 volt.
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