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NEET-XII-Physics

11: Dual Nature Of Radiation And Matter

 page 4
Qstn# 26 Prvs-QstnNext-Qstn
  • #26
    Ultraviolet light of wavelength 2271 Å from a 100 W mercury source irradiates a photo-cell made of molybdenum metal. If the stopping potential is -1.3 V, estimate the work function of the metal. How would the photo-cell respond to a high intensity (∼105 W m-2) red light of wavelength 6328 Å produced by a He-Ne laser?
    Ans : Wavelength of ultraviolet light, λ = 2271 Å = 2271 × 10-10 m

    Stopping potential of the metal, V0 = 1.3 V

    Planck’s constant, h = 6.6 × 10-34 J

    Charge on an electron, e = 1.6 × 10-19 C

    Work function of the metal =

    Frequency of light = ν

    We have the photo-energy relation from the photoelectric effect as:

    = hν - eV0



    Let ν0 be the threshold frequency of the metal.

    ∴= hν0



    Wavelength of red light, = 6328 × 10-10 m

    ∴Frequency of red light,



    Since ν0> νr, the photocell will not respond to the red light produced by the laser.