# NCERT Physics Class 12 Exemplar Ch 11 Dual Nature of Radiation and Matter Part 2

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## Short Answer

**19** Consider Fig. for photoemission.

How would you reconcile with momentum-conservation? Note light (photons) have momentum in a different direction than the emitted electrons.

Ans:

The momentum is transferred to the metal. At the microscopic level, atoms absorb the photon and its momentum is transferred mainly to the nucleus and electrons. The excited electron is emitted.

Conservation of momentum needs to be accounted for the momentum transferred to the nucleus and electrons.

**20** Consider a metal exposed to light of wavelength nm. The maximum energy of the electron doubles when light of wavelength snm is used. Find the work function in eV.

Ans:

Maximum energy

**21** Assuming an electron is confined to a 1nm wide region; find the uncertainty in momentum using Heisenberg Uncertainty principle (Ref Eq 12 of NCERT Textbook). You can assume the uncertainty in position as 1nm. Assuming , find the energy of the electron in electron volts.

Ans:

**22** Two monochromatic beams A and B of equal intensity *I*, hit a screen. The number of photons hitting the screen by beam A is twice that by beam B. Then what inference can you make about their frequencies?

Ans:

The frequency of beam B is twice that of A.

**23** Two particles A and B of de Broglie wavelengths and combine to form a particle C. The process conserves momentum. Find the de Broglie wavelength of the particle C. (The motion is one dimensional).

Ans:

If

**24** A neutron beam of energy *E* scatters from atoms on a surface with a spacing. The first maximum of intensity in the reflected beam occurs at what is the kinetic energy *E* of the beam in?

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