Modern Physics 2 Question 5

5. When a certain photosensitive surface is illuminated with monochromatic light of frequency v, the stopping potential for the photocurrent is V0/2. When the surface is illuminated by monochromatic light of frequency v/2, the stopping potential is V0. The threshold frequency for photoelectric emission is

(Main 2019, 12 Jan II)

(a) 43v

(b) 2v

(c) 3v2

(d) 5v3

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Solution:

  1. Relation between stopping potential and incident light’s frequency is eV0=hfφ0. where, V0 is the stopping potential and φ0 is the the work function of the photosensitive surface.

So, from given data, we have,

eV02=hvφ0

and

eV0=hv2φ0

Subtracting Eqs. (i) from (ii), we have

eV0eV02=hν2hνeV02=hν2eV0=hν

Substituting this in Eq. (i), we get

eV02=eV0φ032eV0=φ0 or 32hν=φ0

If threshold frequency is v0 then

hv0=32hvv0=32v



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