Question

# A certain metallic surface is illuminated with monochromatic light of wavelength, λ. The stopping potential for photo-electric current for this light is 3V0. If the same surface is illuminated with light of wave length 2λ, the stopping potential is V0. The threshold wavelength for this surface for photo- electric effect is :

A

λ/6

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B

6λ

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C

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D

λ/4

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Solution

## The correct option is C 4λ We have, hcλ=W+e(3V0) ….(1) where W is the work function and (3V0) is the stopping potential when monochromatic light of wavelength l is used. Also, hc2λ=W+e V0 ...(2) is used. λ where V0 is the stopping potential when monochromatic light of wavelength 2λ Subtracting equation (2) from equation (1) We get, hc2λ=2e V0 ...(2) ∴V0=hc4eλ Substituting in equation (2) we get, hc2λ=W+eV0=W+hc4λ∴W=hc4λ The threshold wavelength is therefore 4λ .

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