Modern Physics Important Questions

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Very Short Answer Questions [1 Mark Questions]

Ques. If the wavelength of electromagnetic radiation is doubled, what will happen to the energy of photons?

1. Doubled
2. Halved
3. Remains the same
4. Infinite

Ans. The correct answer is b. Halved

Explanation: The energy of a photon is given by E = hf

Where f is the frequency of the electromagnetic radiation and is given by f = c/λ

⇒ E = hc/λ

Since Plack’s constant (h) and speed of light in vacuum (c) is constant, therefore energy of a photon is inversely proportional to the wavelength (λ) i.e.

E ∝ 1/λ

Hence when the wavelength gets doubled, the energy of the photon is reduced to halved.

Ques. What happens to the wavelength of a photon after it collides with an electron?

1. Remains the same
2. Infinite
3. Increases
4. Decreases

Ans. The correct answer is c. Increases

Explanation: A photon delivers some of its energy to the colliding electron, causing its energy to decrease and, as a result, its wavelength increases. This is because the energy of a photon is inversely proportional to its wavelength.

Ques. Laws of physics can be articulated based on _____ according to Einstein’s special theory of relativity.

1. Inertial frame of reference
2. Quantum state
3. Non-inertial frame of reference
4. None of the above

Ans. The correct answer is a. Inertial frame of reference

Explanation: One of the postulates of Einstein's special theory of relativity states that all inertial frames are equivalent to the preparation of physical laws. Therefore, it applies to all inertial frames.

Ques. To derive Einstein's mass-energy relation, the basic principle used is

1. Maxwell's theorem
2. Heisenberg’s principle
3. Work-energy theorem
4. Energy conservation theorem

Ans. The correct answer is c. Work-energy theorem

Explanation: The basic principle used to develop Einstein's mass-energy relationship is the Work-Energy Theorem. It states that the kinetic energy of a moving body is equal to the work done on the body from rest by an external force.

Ques. When a radioactive substance emits an α-particle, its position in the periodic table is lowered by which of the following?

1. One place
2. Two places
3. Three places
4. Four places

Ans. The correct answer is b. Two places

Explanation: The atomic number of a radioactive material reduces by a factor of two when it emits a -particle. As a result, its position in the periodic table is reduced by two places.

Ques. The radiation strikes which part of the photoelectric cell?

1. Anode
2. Cathode
3. Voltmeter
4. None of the above

Ans. The correct answer is b. Cathode

Explanation: The part on which the radiation strikes is the photosensitive plate used as a cathode. When radiation strikes the plate, photoelectrons are ejected and pushed towards the charged plate. This completes the circuit and the current flows.

Ques. Which photon is more energetic: A red one or a violet one?

1. Violet
2. Red
3. Both
4. None of the above

Ans. The correct answer is a. Violet

Explanation: The energy of a photon depends on the frequency of the radiation. The frequency of violet light is more than the frequency of red light, violet is more energetic than red one.

Ques. Which radiations will be most effective for the emission of electrons from a metallic surface?

1. X-rays
2. Microwaves
3. Infrared
4. Ultraviolet

Ans. The correct answer is d. Ultraviolet

Explanation: Ultraviolet radiation is the most effective for photoelectric emission because they have the greatest frequency of all electromagnetic waves, making them the most energetic.

Ques. Inside the Photoelectric cells, which of the following gases are filled?

1. Neon
2. Nitrogen
3. Helium
4. Hydrogen

Ans. The correct answer is a. Neon

Explanation: To increase the photoelectric current due to ionization, photoelectric cells are filled with inert gases such as neon or argon. When the potential difference between the two electrodes exceeds the ionization potential of the gas, the released photoelectrons ionize it. This progressively raises the current's magnitude.

Ques. Which of the following natures of light can help to understand the photoelectric emission phenomenon?

1. The dual nature of light
2. Quantum nature
3. Wave nature of light
4. Particle nature of light

Ans. The correct answer is d. Particle nature of light

Explanation: The photoelectric emission laws were not explained by the wave theory of light. However, Einstein was able to explain photoelectric emission using light's particle nature. When incoming radiation collides with an electron, it is thought that a photon transfers its energy to the electron, resulting in a photoelectric emission phenomenon.

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Short Answer Questions [2 Marks Questions]

Ques. What is modern physics?

Ans. Modern physics is a branch of physics that originated in the early twentieth century and onward, or branches that were significantly affected by early twentieth-century physics. Quantum mechanics, special relativity, and general relativity are important topics in modern physics.

Ques. Who is the father of modern physics?

Ans. Galileo Galilei is often regarded as the "Father of Modern Physics." Albert Einstein is often regarded as the "Father of Classical Physics."

Ques. What is Einstein’s theory of relativity?

Ans. Albert Einstein's theory of relativity is a combination of two theories: his special theory of relativity, proposed in the 1905 paper The Electrodynamics of Moving Bodies, and his theory of general relativity, an expansion of the earlier theory published in 1916 as The Foundation of the General Theory of Relativity. Einstein tried to explain cases in which Newtonian physics would fail to explain observations, and in doing so suggested fundamental changes in human conceptions of time, space, and gravity.

Ques. What is Quantum mechanics?

Ans. Quantum mechanics is a basic physics theory that describes the behavior of nature at the atomic and subatomic particle scales. It is the bedrock upon which all quantum physics is constructed, including quantum chemistry, quantum field theory, quantum technology, and quantum information science.

Ques. What does Modern Physics consist of?

Ans. Modern Physics is the conclusion of two of the twentieth century's most important foundational theories: Einstein's Theory of Relativity and Quantum Mechanics. Quantum mechanics deals with shorter distances, whereas relativity deals with light-year measurements and speeds similar to the speed of light.

Ques. What is black body radiation?

Ans. Blackbody radiation is the energy radiated by any object or system that absorbs all incident radiation. The word is typically used to describe the spectrum of light emitted by any heated object; the filament of a lightbulb and the heating element of a toaster are among the most common.

Ques. What is classical physics?

Ans. It refers to concepts in physics that predate newer, more comprehensive, or more widely applicable theories. Throughout classical physics, energy and matter were considered separate entities.

Ques. What is the uncertainty principle?

Ans. In quantum physics, the uncertainty principle, often known as Heisenberg's indeterminacy principle, is a fundamental concept. According to this theory, some pairs of physical properties, such as position and momentum, cannot be simultaneously known with such precision.

Ques. What is a photon?

Ans. A photon is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation like light and radio waves, and the electromagnetic force's force carrier. Because photons have no mass, they always travel at the speed of light in a vacuum, which is 299792458 m/s.

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Long Answer Questions [3 Marks Questions]

Ques. What is the importance of quantum theory?

Ans. Quantum theory is required to explain physics at the atomic scale. 

• In the case of a constrained system, we may determine its values by measuring its energy and angular momentum. 
• The mathematics-based theory was created by physicists like as Heisenberg, Bohr, Schrödinger, and Dirac. 
• During the late twentieth century, scientists such as Steven Hawking and Richard Feynman contributed to the development of the Quantum Field Theory.

Ques. State the postulates of the special theory of relativity.

Ans. One of the most significant advances in physics is the special theory of relativity. This theory was proposed by Albert Einstein in the form of two postulates.

1. The Principle of Relativity: The states of changing physical systems are unaffected, regardless of whether these state changes are referenced to one or both of two systems in uniform translatory motion relative to one other.
2. The Principle of Invariant Light Speed: Light always travels in space at a constant velocity c that is independent of the state of motion of the emitting body.

Ques. What are the laws of photoelectric emission?

Ans. The following are the laws of photoelectric emission

1. For each particular photosensitive material, there is a minimum cut-off frequency known as the incident radiation threshold frequency, below which no emission occurs regardless of incident radiation intensity.
2. The maximum kinetic energy or equivalent stopping potential of the emitted photoelectrons increases linearly as the frequency of incident radiation increases, provided the frequency of incident radiation is larger than the threshold frequency.
3. The photoelectric current is proportional to the intensity of incident radiation and the frequency of incident radiation for a particular photosensitive material, provided that the frequency of radiation that enters is larger than the threshold frequency.
4. The saturation current is found to be related to the intensity of incident radiation for a particular photosensitive material, but the stopping potential is independent of its intensity.
5. Photoelectric emission is an instantaneous process. Even when the incident radiation is very faint, the time lag between the incidence of radiation and the emission of photoelectrons is very small.

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Very Long Answer Questions [5 Marks Questions]

Ques. What are the important discoveries in modern physics?

Ans. Several experiments contributed to history advancement of Modern Physics. Among these, we can mention those who helped us better understand the structure of matter and atoms. Some examples of significant discoveries are as follows:

• Wilhelm Rontgen discovered the presence of X-rays in 1895. It is a form of invisible, extremely penetrating radiation.
• After a few years, in the year 1900, the German physicist Max Planck stated that energy is charged by an electromagnetic field and has quantized values. It is the integer multiples of a fixed and minimum amount.
• Albert Einstein explained and demonstrated the existence of fast-moving references in his theory of relativity in 1905. This speed was near to the speed of light propagation, allowing us to experience the passage of time and estimate distances in various ways.
• Niels Bohr proposed in 1913 that the energy levels of electrons distributed about atomic nuclei be quantized. It means that their energy is represented by an integer multiple of the minimum quantity.
• The wave-particle duality, proposed by scientist Louis De'Broglie in 1924, demonstrated that anybody may act like a wave.
• Quantum mechanics first appeared in 1926. It was the work of scientists such as Werner Heisenberg and Erwin Schröedinger.

Ques. What are the applications of modern physics?

Ans. The following are the applications of modern physics

• X-ray technology, magnetic resonance imaging, and radiotracing are just a few of the medical technologies that have benefited from modern physics. These medical innovations have proven to be useful in terms of improving human health.
• Nuclear power plants are based on modern physics to produce electricity.
• In the communication sector, optical fiber cables are used for very fast and efficient data transmission based on modern physics.
• Semiconductor devices like transistors and integrated circuits are essential elements in modern electronics. These devices have been explained and designed using quantum mechanics.
• Quantum cryptography uses quantum physics concepts to establish secure communication channels.

Ques. What is the photoelectric effect? The photoelectric effect cannot be explained based on the wave theory of light. Explain this statement.

Ans. When a material absorbs light, it emits electrically charged particles, resulting in the photoelectric effect. The photoelectric effect is the phenomenon of emission of electrons from a metal plate when light of suitable frequency is incident on it.

The phenomenon of the photoelectric effect cannot be explained based on the wave theory of light.

• As per the wave theory, the electric field component (E) of light increases with the increase in the intensity of light.
• Thus the force acting on a free electron when the light incident increases with the increase in the intensity.
• As a result, the kinetic energy acquired by the free electron increases with the increase in intensity of the incident light.
• However, it is observed that the maximum kinetic energy acquired by the photoelectron is the same for both incident beams of different intensities.
• This observation is contradictory to the fact explained by the wave theory of light.

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