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Question -

The work function for thefollowing metals is given:

Na:2.75 eV; K: 2.30 eV; Mo: 4.17 eV; Ni: 5.15 eV. Which of these metals will notgive photoelectric emission for a radiation of wavelength 3300 Å from a He-Cdlaser placed 1 m away from the photocell? What happens if the laser is broughtnearer and placed 50 cm away?



Answer -

Mo and Ni will not showphotoelectric emission in both cases

Wavelength for a radiation, λ =3300 Å = 3300 × 10−10 m

Speed of light, c = 3 × 108 m/s

Planck’s constant, h = 6.6× 10−34 Js

Theenergy of incident radiation is given as:

It can be observed thatthe energy of the incident radiation is greater than the work function of Naand K only. It is less for Mo and Ni. Hence, Mo and Ni will not showphotoelectric emission.

Ifthe source of light is brought near the photocells and placed 50 cm away fromthem, then the intensity of radiation will increase. This does not affect the energyof the radiation. Hence, the result will be the same as before. However, thephotoelectrons emitted from Na and K will increase in proportion to intensity.

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