# Engineering Physics Questions and Answers – Mechanism of Light Emission

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Mechanism of Light Emission”.

1. In X-ray emission tubes, X-ray is emitted by the acceleration of _____
a) Atoms
b) Protons
c) Electrons
d) Neutrons

Explanation: In X-ray emission tubes, the electrons are accelerated by a very high voltage and then decelerated very fast by introducing a metal plate. In this process, the Kinetic energy of electrons is converted into light energy and, thus, X-ray is emitted.

2. Hot bodies emit a specific type of radiation. What are they called?
a) X-rays
d) Visible light

Explanation: The phenomenon of black—body radiations was given by Max Planck. He stated that hot bodies emit radiation over a wide range of wavelengths. An ideal body is the one that emits and absorbs radiation of all frequencies. Such a body called a Black Body and the radiations are called Black body radiations.

3. An iron rod is heated. As it gets heated, it changes color. Which of the following colors indicates a very high temperature?
a) Red
b) Orange
c) White
d) Blue

Explanation: As the body gets hotter, the frequency of the emitted radiation keeps on increasing. Blue color has the highest frequency out of red, orange and white. Thus, as the iron rod gets heated first it would become red, then orange, then white and then finally blue.

4. Which of the following may not be conserved in a photon-particle collision?
a) Total Energy
b) Total momentum
c) Number of photons
d) Number of particles

Explanation: In a photon-particle collision, total energy and the total momentum are conserved. However, the number of photons may vary. Some photons may get absorbed or new photons may be created. Thus, the number of photons may not be conserved in a collision.

5. When an electron jumps from an excited state to ground state it __________
a) Emits a photon having energy 13.6 eV
b) Absorbs a photon having energy 13.6 eV
c) Emits a photon having energy the same as the energy difference between the two states
d) Absorbs a photon having energy the same as the energy difference between the two states

Explanation: Electron in the ground state is extremely stable. Hence, when an electron is in an excited state it tends to come back to its ground state. In this process, a photon is emitted. The Emitted photon has energy equal to the energy difference between the two energy states.

6. Stimulated emission is used for the production of ____________
a) Laser
b) X-rays
c) Gamma rays
d) Infrared rays

Explanation: Stimulated emission is the process in which matter at high energy state is perturbed by a photon having energy equal to the energy difference between the current excited state and a lower state. The matter jumps back to the lower state releasing another photon of the same energy. This process is used for the generation of LASERS.

7. Radioactive decay of the nucleus emits ____________
a) Ultraviolet rays
b) X-Rays
c) Infra-red
d) Gamma rays

Explanation: Gamma rays lie in the upper frequency of the electromagnetic spectrum. It is produced in a nuclear reaction and also emitted in radioactive nuclei. Due to their high penetration power, they are used to destroy cancer cells.

8. What is the frequency of a photon having energy 3.3 X 10-30 J?
a) 4.0 X 1013 Hz
b) 5.0 X 1013 Hz
c) 6.0 X 1013 Hz
d) 7.0 X 1013 Hz

Explanation: Here, E = 3.3 X 10-30 J
Frequency, v =?
We know, Energy = hv, where h is the Planck’s Constant
Therefore, v = E/h
= 3.3 X 10-30J/6.6 X 10-34Js
= 5 X 1013 Hz.

9. The wavelength of a photon is 3000 A. The energy of the photon is ___________
a) 3.47 eV
b) 4.54 eV
c) 6.78 eV
d) 2.12 eV

Explanation: We know, E = hc/λ
= 6.6 X 10-34 X 3 X 108/3 X 10-7
= 6.6 X 10-19
= 3.47 eV.

10. An electron makes a transition from n = 5 state to n = 2 state in the hydrogen atom. What is the frequency of the emitted photon?
a) 4.9 X 1014 Hz
b) 5.9 X 1014 Hz
c) 6.9 X 1014 Hz
d) 7.9 X 1014 Hz

Explanation: The electron makes a transition from n = 5 to n = 2.
ΔE=R$$(\frac{1}{n_1^2} – \frac{1}{n_2^2})$$
= 45.774 X 10-20J
Frequency, v = ΔE/h
= 6.9 X 1014 Hz.

11. Klystron valve and Magnetron valve are used for the production of ___________
b) Micro Waves
c) Infra-red
d) Ultraviolet

Explanation: Microwaves are also called short-wavelength radio waves. They are produced by special vacuum tubes called klystron valve, magnetron valves, and Gunn diodes. They are used in the radar system as they have a short wavelength.

12. Photons are electrically neutral and are not deflected by electric and magnetic fields.
a) True
b) False

Explanation: Photons are considered to have no charge. They are simply quanta of energy. They are not affected by the presence of the electric and magnetic fields.

13. What will be the energy of the photon emitted in the following case? a) 34.15 eV
b) 6.3 eV
c) 27.3 eV
d) 13.15 eV

Explanation: Whenever an electron makes a transit from an excited state to a lower state, the photon released has the energy equal to the energy difference between the two states. In this case, the Energy difference = E4 – E2 = 27.3 eV. 