What is Modern Physics?
The branch of physics that deals with Newtonian concepts is known as modern physics. It is grounded on the two major discoveries of the twentieth century: Quantum Mechanics and Relativity.
Modern physics often includes new theories that are different from the classical descriptions and an advanced explanation of nature through them. Also, it involves Einstein’s relativity and elements of quantum mechanics. For example, quantum effects classically contain distances related to atoms. Relativistic effects, on the other hand, naturally include velocities related to the speed of light.
Read More:Photoelectric Effect
Who is the Father of Modern Physics?
The 20th-century scientist Albert Einstein is well-known as the father of modern physics. He came up with the most significant and innovative ideas. One of the most significant works of Albert Einstein is the famous equation E=mc2 and the theory of relativity.
For his accomplishments in theoretical physics, mainly discovering the laws of the photoelectric effect, he was awarded the Nobel Prize for physics.
Important Questions on Modern Physics
1) Laws of physics can be articulated based on _____ according to Einstein’s special theory of relativity.
a) Quantum state
b) Non-inertial frame of reference
c) Inertial frame of reference
d) None of these
Correct Option: (c)
Explanation: All the inertial frames are equal to the preparation of laws of physics; this was stated in one of the postulates of Einstein’s special theory of relativity states. Therefore, it is appropriate for all inertial frames.
2) Which among the following frame of reference has four coordinates: x, y, z, and t?
a) Space-time reference
b) Quantum frame of reference
c) Inertial frame of reference
d) 4-dimensional plane
Correct Option: (a)
Explanation: Space-time frame is a frame that consists of four coordinates of x, y, z and time t. Einstein’s special theory of relativity space-time frame plays a major role in the concept of space-time frame.
3) To obtain mass-energy relation, the basic theorem used was _____.
a) Heisenberg’s principle
b) Maxwell theorem
c) Energy conservation theorem
d) Work-energy theorem
Correct Option: (d)
Explanation: The basic principle used to derive Einstein’s mass-energy relation is the Work-energy theorem. It states that the work done by the external force on the moving body from rest is equal to the kinetic energy of that body.
4) What changes are observed during Einstein’s photoelectric experiment, when we increase the frequency of the incident radiation?
a) Saturation current value increases
b) Stopping potential value decreases
c) Stopping potential value increases
d) No effect
Correct Option: (c)
Explanation: The kinetic energies of the emitted electron increase when the frequency of the incident radiation increases. Therefore, more repulsive force is required to stop them, which results in an increase in stopping potential. But as the intensity of the incident radiation increases, the value of saturation current increases, due to which the frequency decreases and the stopping potential value also decreases.
5) What is the time interval between the ejection of photoelectrons and the incidence of photons?
a) Greater than 10-6 s
b) Between 10-6 s and 10-10 s
c) Less than 10-9 s
d) 10 second
Correct Option: (c)
Explanation: Practically, there is no time lag (< 10-9 s) between the incident photon and the emission of an electron. Photoelectric emission is a rapid process because the photoelectric emission happens due to the elastic collision between a photon and an electron.
6) How is the photoelectric current affected by the intensity of incident light?
a) Photoelectric effect increases as intensity increases
b) Photoelectric effect decreases as the intensity increases
c) Photoelectric effect becomes twice, as the intensity decreases
d) No effect
Correct Option: (a)
Explanation: The number of photoelectrons emitted depends on the number of photons falling on the metal surface because each incident photon ejects one photoelectron from a metal surface. This process eventually depends on the intensity of the incident light. Therefore, the number of photoelectrons ejected increases as the intensity increases, and hence photoelectric current increases.
7) Which of the following nature of light can help to understand the photoelectric emission phenomenon?
a) Quantum nature
b) Dual nature of light
c) Particle nature of light
d) Wave nature of light
Correct Option: (c)
Explanation: The wave theory of light failed to explain the laws of photoelectric emission. But Einstein was able to explain the photoelectric emission using the particle nature of light. It was considered that a photon transfers its energy to the electron when its incident radiation collides with an electron and thus causes a photoelectric emission phenomenon.
8) Inside the Photoelectric cells, which of the following gases are filled?
a) Hydrogen
b) Nitrogen
c) Helium
d) Neon
Correct Option: (d)
Explanation: To increase the photoelectric current due to ionization, inert gases like neon or argon are filled inside the photoelectric cells. The emitted photoelectrons ionize the gas, when the potential difference between the two electrodes surpasses the ionization potential of the gas. This eventually increases the magnitude of the current.
9) The radiation strikes on which part of the photoelectric cell?
a) Cathode
b) Voltmeter
c) Anode
d) None of these
Correct Option: (a)
Explanation: The photosensitive plate which is used as a cathode is the part on which the radiation strikes. Photoelectrons are ejected, which are accelerated towards the charged plate when radiation falls on the plate. This completes the circuit and the current flows.
10) When a photon collides with an electron, what happens to its wavelength?
a) Decreases
b) Increases
c) Infinite
d) None of these
Correct Option: (b)
Explanation: To the colliding electron, a photon transfers a part of its energy, so its energy reduces, and accordingly, its wavelength increases. The reason behind it is that the energy of a photon is inversely proportional to the wavelength of a photon.
Practice Questions
1) What is black body radiation?
2) What is Einstein’s theory of relativity?
3) What is the photoelectric effect?
4) Who is the father of modern physics?
5) What do you mean by a photon?
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