Chapter 11: Dual Nature of Radiation and Matter Short Questions | physics Class 12 CBSE

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Chapter 11: Dual Nature of Radiation and Matter Short Questions | physics Class 12 CBSE | ByteNotes.in

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Question 1

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Define work function of a metal and state its unit.

Work function is the minimum energy required by an electron to escape from the surface of a metal against the attractive pull of the positive ions.
It is denoted by φ₀ and is measured in electron volts (eV).
The work function depends on the properties of the metal and the nature of its surface, and it varies from metal to metal.

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Question 2

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What is thermionic emission? How does it differ from photoelectric emission?

Thermionic emission is the process of electron emission from a metal surface by supplying sufficient thermal energy through heating the metal.
Photoelectric emission occurs when light of suitable frequency falls on the metal surface, causing electrons to be emitted by absorbing photon energy.
In thermionic emission the energy source is heat, while in photoelectric emission the energy source is incident light of frequency above the threshold value.

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Question 3

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State the observations made by Hallwachs using a negatively charged zinc plate connected to an electroscope when exposed to ultraviolet light.

The negatively charged zinc plate lost its charge when illuminated by ultraviolet light, indicating emission of negative particles from its surface.
An uncharged zinc plate acquired a positive charge upon irradiation with ultraviolet light, confirming emission of negatively charged particles.
The positive charge on a positively charged zinc plate was further enhanced under ultraviolet illumination, supporting the conclusion that negatively charged particles (electrons) were being emitted.

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Question 4

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Define stopping potential. How is it related to the maximum kinetic energy of photoelectrons?

Stopping potential (V₀) is the minimum negative (retarding) potential applied to the collector plate at which the photocurrent becomes exactly zero.
It represents the potential sufficient to stop even the most energetic photoelectrons from reaching the collector.
The maximum kinetic energy of the emitted photoelectrons is directly related to stopping potential by the equation Kmax = eV₀.

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Question 5

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How does the photocurrent vary with the intensity of incident radiation in the photoelectric effect?

The photocurrent increases linearly with the intensity of incident radiation when the frequency is kept above the threshold and potential is kept constant.
This is because the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation.
Higher intensity means more photons incident per second, leading to more electron absorptions and hence greater photocurrent.

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Question 6

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Define threshold frequency. Why does it differ for different metals?

Threshold frequency (ν₀) is the minimum frequency of incident radiation below which no photoelectric emission occurs from a metal surface, regardless of the intensity of light.
It is given by ν₀ = φ₀/h, where φ₀ is the work function of the metal and h is Planck's constant.
Since the work function is a property of the metal that depends on the nature of the material and its surface, the threshold frequency is different for different metals.

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Question 7

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Explain why the classical wave theory of light fails to explain the existence of threshold frequency in the photoelectric effect.

According to wave theory, the energy of incident radiation depends on its intensity and is continuously distributed over the wavefront, so any frequency of light should eventually supply enough energy for emission.
Therefore, wave theory predicts that with sufficiently intense radiation, photoelectric emission should occur at all frequencies, with no minimum threshold frequency required.
This directly contradicts the experimental observation that no emission occurs below the threshold frequency ν₀ regardless of how intense the incident radiation is.

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Question 8

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State two properties of photons as described in the photon picture of electromagnetic radiation.

Each photon has energy E = hν and momentum p = hν/c, where ν is the frequency of the radiation and c is the speed of light.
Photons are electrically neutral and are not deflected by electric or magnetic fields.
The energy of a photon depends only on its frequency, not on the intensity of radiation; increasing intensity only increases the number of photons per second, not the energy of each photon.

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Question 9

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How does stopping potential vary with the frequency of incident radiation? What does this imply about the energy of photoelectrons?

Stopping potential increases linearly with the frequency of incident radiation for a given photosensitive material, as shown by the straight-line graph of V₀ versus ν.
This implies that the maximum kinetic energy of the emitted photoelectrons increases with the frequency of incident light.
The higher the frequency, the more energetic the photoelectrons, requiring a greater retarding potential to bring the photocurrent to zero.

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Question 10

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Why does the stopping potential remain the same even when the intensity of incident radiation is increased, keeping frequency constant?

Stopping potential is related to the maximum kinetic energy of the emitted photoelectrons, which depends on the energy of each individual photon.
The energy of a photon depends only on its frequency (E = hν), not on the intensity of radiation. Increasing intensity only increases the number of photons, not their individual energy.
Since no individual electron gains more energy from a photon when intensity increases, the maximum kinetic energy and hence the stopping potential remain unchanged.

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Question 11

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What is saturation current in a photoelectric experiment and under what condition is it achieved?

Saturation current is the maximum value of the photoelectric current obtained when all the photoelectrons emitted by the emitter plate reach the collector plate.
It is achieved when the accelerating potential of the collector plate is sufficiently high so that no emitted electron is lost on its way to the collector.
Further increasing the accelerating potential beyond this point does not increase the photocurrent, since all available photoelectrons are already being collected.

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Question 12

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State Einstein's photoelectric equation and identify what each term represents.

Einstein's photoelectric equation is Kmax = hν − φ₀, where Kmax is the maximum kinetic energy of the emitted photoelectron.
Here, hν is the energy of the incident photon with h as Planck's constant and ν as the frequency of incident radiation.
φ₀ is the work function of the metal, representing the minimum energy needed for the electron to escape from the metal surface.

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Question 13

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How did Millikan's experiments between 1906–1916 contribute to the validation of Einstein's photoelectric equation?

Millikan performed precise experiments on the photoelectric effect aiming to disprove Einstein's equation, but his measurements of the stopping potential versus frequency for alkali metals like sodium confirmed a linear relationship.
By measuring the slope of the V₀ versus ν graph and using the known value of charge e, Millikan accurately determined Planck's constant h, which matched the value obtained from independent experiments.
In 1916, Millikan thus proved the validity of Einstein's photoelectric equation over a wide range of radiation frequencies and for several alkali metals, instead of disproving it.

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Chapter 11: Dual Nature of Radiation and Matter | Short Questions

Define work function of a metal and state its unit.

What is thermionic emission? How does it differ from photoelectric emission?

State the observations made by Hallwachs using a negatively charged zinc plate connected to an electroscope when exposed to ultraviolet light.

Define stopping potential. How is it related to the maximum kinetic energy of photoelectrons?

How does the photocurrent vary with the intensity of incident radiation in the photoelectric effect?

Define threshold frequency. Why does it differ for different metals?

Explain why the classical wave theory of light fails to explain the existence of threshold frequency in the photoelectric effect.

State two properties of photons as described in the photon picture of electromagnetic radiation.

How does stopping potential vary with the frequency of incident radiation? What does this imply about the energy of photoelectrons?

Why does the stopping potential remain the same even when the intensity of incident radiation is increased, keeping frequency constant?

What is saturation current in a photoelectric experiment and under what condition is it achieved?

State Einstein's photoelectric equation and identify what each term represents.

How did Millikan's experiments between 1906–1916 contribute to the validation of Einstein's photoelectric equation?

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