Physics Class 12 NCERT Solutions: Chapter 11 Dual Nature of Radiation and Matter Part 12 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Graph in Photoelectric Effect

Q: 26. Ultraviolet light of wavelength from a mercury source irradiates a photo-cell made of molybdenum metal. If the stopping potential is , estimate the work function of the metal. How would the photo-cell respond to a high intensity red light of wavelength produced by a He-Ne laser?


Wavelength of ultraviolet light,

Stopping potential of the metal,

Planck՚s constant,

Charge on an electron,

Work function of the metal

Frequency of light

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

Let be the threshold frequency of the metal.

Wavelength of red light,

∴ Frequency of red light,

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

Q: 27. Monochromatic radiation of wavelength from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be . The source is replaced by an iron source and its line irradiates the same photo-cell. Predict the new stopping voltage.


Wavelength of the monochromatic radiation,

Stopping potential of the neon lamp,

Charge on an electron,

Planck՚s constant,

Let be the work function and be the frequency of emitted light.

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

Wavelength of the radiation emitted from an iron source,

Let be the new stopping potential. Hence, photo-energy is given as:

Hence, the new stopping potential is .

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