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Question

When a point light source, of power W emitting light of wavelength λ is kept at a distance 'a' from a photosensitive surface of work function ϕ and area S, we will have


A

Number of photons striking the surface per unit time as

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B

the maximum energy of the emitted photoelectrons as

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C

The stopping potential needed is for most energetic emitted photoelectrons.

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D

Photoemission occurs only for range,

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Solution

The correct options are
A

Number of photons striking the surface per unit time as


B

the maximum energy of the emitted photoelectrons as


D

Photoemission occurs only for range,


We know Intensity at a distance 'a' from the source,

I = P4πa2

Let the number of photon hitting the surface 'S' per unit time be n.

(P4πa2)S=nhcλ

n=WSλ4πa2hc

The maximum energy of emitted photo electron is, Emax=hvϕ=hcλφ=(hcλϕ)λ

e.Vs=Emax

Hence Vs=Emax=(hcλϕ)eλ

For Photoemission to be possible

we must have hvϕ. Hence, hcλφ(or)λhcφ,

Thus, the permitted range of value of λ is 0λhcφ

Hence the correct choices (a),(b) and (d)


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