Quantum Physics Interview Questions & Answers

Question 1. Why ”quantum Physics” Is Named As It Is?

Answer :

In 1905 Albert Einstein explained the photoelectric phenomena by assuming that light can be absorbed in certain “packets”, only. He suggested that light has an elementary “quanta”; the photon, as it was then called. This contributed to the birth of a new physics in an important way. Many other quantities (that were previously considered “continuous”) were also discovered to be quantized. Thus the emerging new physics was named “quantum physics”.

Question 2. So The Essence Of Quantum Physics Is That Everything Has A Quanta?

Answer :

Not really. In fact, it is not even true. For example, if we take an H-atom, we find that it has certain energy levels. But it is not true that “energy is quantized”. If we now take a different atom, we find different energy levels; the actual levels does not reflect some universal property of energy — rather, it is specific to the system in question. By the way, such things can happen in classical physics, too. For example, if we have a cord, it can only vibrate at certain frequencies. However, different cords can vibrate at all sort of different frequencies — altogether, in classical physics there is no natural unit of frequency.

Question 3. Then What Is The Essence Of Quantum Physics? What Makes It So Different From Classical Physics?

Answer :

Quantum physics takes account of the uncertanity present in nature. (By the way, you should also note, that quantum physics is not a single theory; rather, it is a general framework. More specifically, one talks about quantum mechanics, quantum thermodynamics, quantum field theory, etc.) Here the word “uncertanity” is not meant in the sense that we don’t know something (so that we would be uncertain of something).

Quantum physics claims that reality isn’t something crystal clear; instead, it is somewhat misty. When we describe the electron’s position in an H-atom by a certain spherical “cloud”, we do so not because we are not sure where it is (which would be a simple lack of information on the observer’s side). Rather, the electron itself is not sure about its position (“intrinsic uncertanity”), and in some sense it is really both here and there and a little bit all around.

Question 4. So Quantum Physics Must Use Probability Theory?

Answer :

Yes, but it uses a “built in” probability theory which is different from the classical one. There is actually a mathematical difference between probabilities arising from lack of knowledge and intrinsic uncertanity. When we use classical probability theory, we tacitly assume that at each experimental round, each measurable quantity (described in the theory by a random variable) assumes a value — independently from the fact whether we have measured it or not. In reality, at each experimental round we can only measure some quantities.

It turns out that the statistics emerging from experimental data actually contradicts the assumption that at each experimental round, all quantities had a value (and that only we did not know them). On the other hand, the probability theory used in quantum physics does not make such assumptions and in fact the predictions made by using quantum physics are in perfect agreement with experimental data. From the point of view of abstract mathematics, the main difference is that the event-lattice used in classical probability theory is distributive, whereas the one used by quantum physics isn’t.

Question 5. I’ve Heard That In Quantum Physics A Lot Of Fancy Mathematical Objects Like Hilbert Spaces Are Used, And That In Particular, Measurable Quantities Are Described By Self-adjoint Operators. Are These Things Related To What You Have Just Explained?

Answer :

Yes, these are mathematical elements of the “built in” probability theory used by quantum physics.

Question 6. How About The Particle-wave Duality?

Answer :

It is just another example of uncertainty. Consider light, which was already mentioned in the beginning of our discussion. It can only be emitted and absorbed in certain units; this is what have suggested the photon-theory.

Yet to describe its propagation one is forced to talk about waves. (Even if we deal with a single photon!) Actually, this is true not only for light: it is a general fact regarding every elementary particle. So from the classical point of view, the situation is rather paradoxical: a particle can sometimes behave like a wave. From the point of quantum physics, there is no paradox. The particle is a particle, but its position is uncertain. It can be both here and little bit also there, so actually even a single particle can produce interference phenomena.

Question 7. Why Is It That Sound Waves Are Not Normally Considered As Having Particle-like Properties, Nor Raindrops As Having Wave-like Properties?

Answer :

The wavelength of a wave is related to its momentum and the Planck constant by the equation l=h/mv. For sound waves, their wavelength is too long hence the sound particles have too little momentum to exhibit particulate properties. For raindrops, due to their large mass and hence momentum, their wavelength is too short for them to undergo significant diffraction. Diffraction can only be observable if the dimension of the aperture is comparable to the wavelength of the wave.

Question 8. Explain Qualitatively The Phenomenon Of Quantum Tunneling Of An Electron Across A Potential Barrier?

Answer :

An electron is considered as a wave function. The probability of finding an electron is directly proportional to the square of the amplitude of the wave function. When the wave function of an electron encounters a potential barrier, its amplitude decreases exponentially. For a narrow barrier, the wave amplitude may not become zero after the electron passes through the barrier. Hence, there is a non-zero probability that the electron will be found beyond the barrier. This process is called quantum tunneling.

Question 9. Explain The Different Parts Of The X-ray Radiation Intensity Graph?

Answer :

The broad continuous spectrum:

Formation: Electrons emitted by the heated filament are made to accelerate through a high PD before they collide with the metal target with very high speeds à interactions with the nuclei of the target atoms, thus electrons lose KE à KE lost converted to energy of x-ray photos radiated from the target; different electrons slowed to different extent à energies of x-ray photons produced take a continuous of values à continuous spectrum formed.

The sharp characteristic peak (unique for each element):

• Occurs when bombarding electron colliding with a target atom has enough energy to remove an inner-shell electron from the atom.
• Existence of Ka and Kb values: Incoming electron knocked off an electron in the n = 1 level (K-shell), in which the vacancy in this shell is then filled by an electron from the n = 2 L-shell, an x-ray photon of the Ka characteristic x-ray is emitted; For Kb, when the vacancy in the K-shell is filled by an electron dropping from the n = 3 M-shell, x-ray photon of the Kb characteristic x-ray is emitted.
• Why is the intensity of the Ka characteristic x-ray > Kb characteristic x-ray: Electrons in the n = 2 L-shell are nearer to the n = 1 K-shell, thus there is a greater probability that the vacancy in the K-shell is filled by an electron from the L-shell than the n =3 M-shell.

Other points to note for X-rays:

• Same target material –> characteristic x-rays produced have same wavelengths –> energy levels of target atoms are the same.
• Higher voltage applied in x-ray tube –> minimum wavelength of x-rays produced is lower –> bombarding electrons produced by tube have higher initial KE (ß with higher voltage applied).

Question 10. Describe And Interpret Qualitatively The Evidence Provided By Electron Diffraction For The Wave Nature Of Particles.

Answer :

When a beam of electrons passed through a thin film of crystal, the dispersion pattern of the emergent electrons produced on a screen is observed to be similar to the diffraction pattern produced by a beam of X-ray. This phenomenon provides evidence for the wave nature of particles like electrons.

Question 11. Distinguish Between Emission And Absorption Line Spectra.

Answer :

An emission line spectrum of an element consists of coloured lines on a dark background while an absorption spectrum consists of dark lines on a coloured background at the same discrete wavelength positions for the same element. For emission spectra, electrons transit from a higher energy level to a lower energy level. For absorption spectra, electrons transit from a lower energy level to a higher energy level.

Question 12. Explain How Spectral Lines Show Discrete Energy Levels In An Atom.

Answer :

An emission spectrum consists of a set of discrete wavelengths. A photon is emitted from an isolated atom when one of its electrons transits from a higher to a lower energy level. Energy of the photon is equal to the energy difference between the two levels involved in the transition.

Question 13. What Are The 4 Results Of The Photoelectric Effect Experiments?

Answer :

1. Current is proportional to intensity. This result can be explained using wave nature and particulate nature of light.
2. For every material of cathode irradiated, there is a threshold frequency below which no electrons would be emitted from the cathode regardless of light intensity. This result can be explained using the particulate nature of light only.
3. The maximum kinetic energy of emitted photoelectrons depends only on the frequency of the incident radiation, and not its intensity. This result can be explained using the particulate nature of light only.
4. The emission of photoelectrons starts with no observable time lag, even for very low intensity of incident radiation. This result can be explained using the particular nature of light only.

Question 14. What Is Black Body And What Are Its Characteristics?

Answer :

• A perfect black body is the one which absorbs and also emits the radiations completely.
• In practice nobody is perfectly black. We have to coat the black color over the surface to make a black body.
• Black body is said to be a perfect absorber, since it absorbs all the wavelengths of the incident radiation. The black body is a perfect radiator, because it radiates the entire wavelength absorbed by it. This phenomenon is called black body radiation.

Question 15. What Are The Applications Of Schrödinger Wave Equations?

Answer :

• It is used to find the electrons in the metal.
• It is used to find the energy levels of an electron in an infinite deep potential well.

Question 16. What Is Meant By Energy Spectrum Of A Black Body? What Do You Infer From It?

Answer :

The distribution of energy for various wavelengths at various temperatures is known as energy spectrum of a black body.

INFERENCE:

• When temperature increases, the wavelength decreases.
• The total energy emitted at any particular temperature can be found with the help of the area traced by the curve.

Question 17. Mention The Applications Of Electron Microscope.

Answer :

• It has a very wide area of applications in the field of biology, metallurgy, physics, chemistry, medicine, and engineering.
• It is used to determine the complicated structures of crystals.
• It is used in the study of celluloid’s.
• It is used to study the structure of microorganisms such as virus, bacteria etc.

Question 18. What Is Meant By Degenerate And Non-degenerate State? Give Examples.

Answer :

Degenerate state: for various combinations of quantum numbers if we get same Eigen value (Energy levels) but different Eigen functions, then it is called degenerate state.

Non- degenerate state: for various combinations of quantum numbers if we get same Eigen values (Energy levels) and same Eigen functions, then it is called Non- Degenerate state.

Question 19. What Is Physical Significance Of Wave Function?

Answer :

• The probability of finding a particle in space, at any given instant of time is characterized by a function Ψ(x, y, z) called wave-function.
• It relates the particle and the wave statistically.
• It gives the information about the particle behavior.
• It is a complex quantity.
• |Ψ 2| represents the probability density of the particle, which is real and positive.

Question 20. Explain Planck’s Hypothesis Or What The Postulates Of Planck’s Quantum Theory? (or) What Are The Assumptions Of Quantum Theory Of Black Body Radiation?

Answer :

The electrons in the black body are assumed as simple harmonic oscillators.
The oscillators will not emit energy continuously.
They emit radiation in terms of quanta of magnitude ‘hγ’, discretely.

E = nhγ where n= 1, 2, 3, 3,….