 # CBSE Class 12 Physics NCERT Exemplar Solutions: Chapter 11 – Dual Nature of Radiation and Matter

May 30, 2018 16:59 IST CBSE Class 12 Physics NCERT Exemplar Solutions

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In this article, we bring to you NCERT Exemplar Solutions for all the questions of Class 12 Physics Chapter 11 – Dual Nature of Radiation and Matter. It will definitely help you to score good marks in both board exams and competitive exams. These solutions are explained by experienced Subject Experts of Physics.

Important Topics of Chapter 11 – Dual Nature of Radiation and Matter:

1. Photoelectric effect

2. Hertz and Lenard’s observations

3. Einstein photoelectric equation

4. de Broglie hypothesis

5. Davisson-Germer experiment

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Types and number of questions in this chapter:

 Types Number of questions MCQ I (Multiple choice questions with single correct option) 8 MCQ II (Multiple choice questions with multiple correct options) 5 VSA (Very short answer type questions) 5 SA (Short answer type questions) 6 LA (Long answer type questions) 5 Total 29

Few problems and their solutions from this chapter are given below:

Question:

Consider a beam of electrons (each electron with energy E) incident on a metal surface kept in an evacuated chamber. Then,

(a) No electrons will be emitted as only photons can emit electrons

(b) Electrons can be emitted but all with an energy, E

(c) Electrons can be emitted with any energy, with a maximum of E − Φ (Φ is the work function)

(d) Electrons can be emitted with any energy, with a maximum of E

Solution:(d)

When a beam of electrons of energy E is incident on a metal surface kept in an evacuated chamber due to elastic collision electrons can be emitted with maximum energy E. When part of incident energy of electron is used in liberating the electrons from the surface of metal, then electrons can be emitted with energy less than E.

Question:

A particle moves in a closed orbit around the origin, due to a force which is directed towards the origin. The de-Broglie wavelength of the particle varies cyclically between two values λ1 λ2 with λ1 > λ2. Which of the following statement are true?

(a) The particle could be moving in a circular orbit with origin as centre

(b) The particle could be moving in an elliptic orbit with origin as its focus

(c) When the de-Broglie wavelength is λ1 the particle is nearer the origin than when its value is λ2

(d) When the de-Broglie wavelength is λ2, the particle is nearer the origin than when its value is λ1

Solution: (b, d)

If particle is moving in an elliptical orbit with origin as its one focus, then the de-Broglie wavelength of the particle can be varying cyclically between two values λ1 and λ2. Let the speed of particle at A and B be v1 and v2 respectively and origin is at focus O.

Also, let us suppose λ1 and λ2 are the de-Broglie wavelengths associated with particle while moving at A and B respectively. Then, We are given that, λ1 > λ2

Using (3), we get

v1 > v2

According to law of conservation of angular momentum, the particle moves faster when it is closer to focus. From the figure given above, we note that origin O is closed to P than A.

Question:

There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength.

Solution:

If the materials absorb photons of shorter wavelength, then the energy of the incident photon on the material is high and the energy of emitted photon is low when it has a longer wavelength.

But, the energy of the incident photon is low for the substances which have to absorb photons of larger wavelength. Similarly, the energy of emitted photon is high to emit light of shorter wavelength. So, according to this statement material has to supply the energy for the emission of photons.

For a stable substance, it can’t be possible.

Question:

Do all the electrons that absorb a photon come out as photoelectrons?

Solution:

In photoelectric effect, most of the electrons get scattered into the metal by absorbing a photon. Therefore, all the electrons that absorb a photon don’t come out as photoelectron.

Question:

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

The momentum of incident photon is transferred to the metal during the photoelectric emission. The atoms of a metal absorb the photon and its momentum is transferred mainly to the nucleus and electrons. The excited electron is emitted.

Therefore, the conservation of momentum is to be considered as the momentum of incident photon transferred to the nucleus and electrons.

CBSE Class 12 Physics NCERT Exemplar Solutions: Chapter 10 – Wave Optics

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