when a point light source, of power P emitting monochromatic light of wavelengthλ is kept at a distance α from a photo-sensitive surface of work-function W0 and area A which of the expressions is wrong

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number of photons striking the surface per unit time as PλA/4πcha2

the stopping potential needed to stop the most energetic emitted photo-electrons as hc−λW0/λe

photo emission only if λ lies in the range 0≤λ≤hc/W0

none

answer is D.

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Detailed Solution

The number of photons released per unit time is given byn= Power energy of photon =P(hc/λ) These photons are spread out in all directions over an ara 4πa2 The share of an area ,4 is a fraction A/4πa2 of total number of photonc emitted '. number of photons strilJng the surface per unit time =PAλ/4πhca2 The maximum energy of the emitted photo-electron is Emax=hv−W0=hcλ−W0 =1λhc−λW0 Further eVs=Emax or Vs=Emaxe=1λehc−λW0 where Ys = stopping potential. For photo emission to be possible, we must have h∨≥W0 hcλ≥W0 or λ≤hcW0 So, permitted range of values of λ is 0≤λ≤hc/W0

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