# Physics Class 12 NCERT Solutions: Chapter 11 Dual Nature of Radiation and Matter Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 7. A sodium lamp radiates energy uniformly in all directions. The lamp is located at the centre of a large sphere that absorbs all the sodium light which is incident on it. The wavelength of the sodium light is . (A) What is the energy per photon associated with the sodium light? (B) At what rate are the photons delivered to the sphere?

Power of the sodium lamp,

Wavelength of the emitted sodium light,

Planck՚s constant,

Speed of light,

(A) The energy per photon associated with the sodium light is given as:

(B) Number of photons delivered to the sphere

The equation for power can be written as:

Therefore, every second, photons are delivered to the sphere.

Q: 8. The threshold frequency for a certain metal is . If light of frequency is incident on the metal, predict the cutoff voltage for the photoelectric emission.

Threshold frequency of the metal,

Frequency of light incident on the metal,

Charge on an electron,

Planck՚s constant,

Cut-off voltage for the photoelectric emission from the metal

The equation for the cut-off energy is given as:

Therefore, the cut-off voltage for the photoelectric emission is

Q: 9. The work function for a certain metal is . Will this metal give photoelectric emission for incident radiation of wavelength ?

No

Work function of the metal,

Charge on an electron,

Planck՚s constant,

Speed of light,

The energy of the incident photon is given as:

It can be observed that the energy of the incident radiation is less than the work function of the metal. Hence, no photoelectric emission will take place.

Q: 10. Light of frequency is incident on a metal surface. Electrons with a maximum speed of are ejected from the surface. What is the threshold frequency for photoemission of electrons?

Frequency of the incident photon,

Maximum speed of the electrons,

Planck՚s constant,

Mass of an electron,

For threshold frequency, the relation for kinetic energy is written as:

Therefore, the threshold frequency for the photoemission of electrons is

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