CBSE Class 12 Physics Notes Chapter 11 Dual Nature of Radiation and Matter

Maxwell's equations and Hertz's experiments established light's wave nature in electromagnetism. Investigations in the late 19th century on electric discharge through low-pressure gases led to significant discoveries.

• Roentgen discovered X-rays in 1895, followed by J. J. Thomson's discovery of the electron in 1897.
• William Crookes discovered cathode rays in 1870, suggesting they consisted of fast-moving negatively charged particles.
• Applying electric and magnetic fields, Thomson determined cathode ray speed and specific charge.
• Thomson found cathode ray particles' charge-to-mass ratio was independent of material or gas used.
• Certain metals emitted negatively charged particles when irradiated by ultraviolet light or heated, matching cathode ray properties.
• Thomson named these particles electrons in 1897, proposing they were fundamental constituents of matter.
• In 1913, Millikan's experiment measured the charge on an electron, establishing electric charge quantization.
• From charge and specific charge values, the mass of the electron was determined.

Dual Nature of Radiation and Matter Notes are important for CBSE Class 12 Physics exam preparation as the chapter has a weightage of 12 marks along with the chapters Atoms and Nuclei in the Class 12 Physics exam 2024.

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Class 12 Physics Chapter 11 Notes – Dual Nature of Radiation and Matter

Electron Emission

Free electrons can be emitted from a metal surface through thermionic emission, field emission, secondary emission, or photoelectric emission.

• Thermionic Emission: Heating the metal provides thermal energy to free electrons, allowing them to escape.
• Field Emission: Applying a strong electric field, around 108 V/m, induces electron emission, as seen in spark plugs.
• Secondary Emission: The process of emission due to the incident of energetic electron beam.
• Photoelectric Emission: When light of appropriate frequency illuminates a metal surface, electrons are emitted, known as photoelectrons.

Electron Emission

Photoelectric Effect

Hertz’s Observations

• Heinrich Hertz discovered photoelectric emission during his electromagnetic wave experiments in 1887.
• High voltage sparks across a detector loop increased when the emitter plate was illuminated by ultraviolet light.
• Light facilitated the escape of free electrons from the metal surface.

Hertz’s Observations

Hallwachs’ And Lenard’s Observations

• Wilhelm Hallwachs and Philipp Lenard investigated photoelectric emission during 1886-1902.
• Lenard observed that ultraviolet radiations caused current flow when falling on the emitter plate of an evacuated glass tube.
• Hallwachs found that zinc plates lost charge when illuminated by ultraviolet light, indicating emission of negatively charged particles.

Experimental Study Of Photoelectric Effect

A schematic glass tube contains a photosensitive plate (emitter) and a collector plate, with monochromatic light irradiating the emitter.

• Electrons emitted from the photosensitive plate are captured by the collector plate, generating electric current in the circuit.
• The voltmeter regulates the potential difference between the emitter and collector plates, while the microammeter measures the resulting photo current.
• Photocurrent changes are analyzed concerning (a) radiation intensity, (b) incident radiation frequency, (c) plate potential difference, and (d) emitter plate material.
• Light intensity and frequency can be modified by changing the light source distance or using colored filters.

Experimental Study Of Photoelectric Effect

Effect Of Intensity Of Light On Photocurrent

• Collector A maintains a positive potential relative to emitter C to attract ejected electrons from C.
• When the frequency and accelerating potential remain constant, varying light intensity results in a linear increase in photocurrent.
• This indicates a direct proportionality between the number of emitted photoelectrons and incident radiation intensity.

Effect Of Potential On Photoelectric Current

• Increasing the positive potential of plate A leads to a rise in photocurrent until it reaches saturation, where all emitted photoelectrons are collected by the plate A.
• Applying a negative potential to plate A results in a decrease in photocurrent until it reaches zero at the stopping potential (V₀), indicating the energy required to repel even the most energetic photoelectrons.

Effect Of Frequency Of Incident Radiation On Stopping Potential

• The stopping potential (V₀) varies linearly with the frequency of incident radiation for a given photosensitive material.
• There exists a minimum cut-off frequency (ν₀) below which no emission of photoelectrons occurs, regardless of the incident light intensity. 
• Above ν0, the stopping potential increases linearly with frequency, indicating greater kinetic energy for emitted electrons.

Photoelectric Effect And Wave Theory Of Light

• The wave theory of light predicts that the intensity of radiation should increase the maximum kinetic energy of photoelectrons and eliminate the need for a threshold frequency. 
• However, these predictions contradict observations regarding photoelectric emission.
• According to the wave theory, energy absorption by electrons occurs continuously over the entire wavefront of radiation. 
• However, calculations suggest that it would take hours or more for a single electron to absorb enough energy to overcome the work function, contradicting the instantaneous nature of photoelectric emission observed (iv).
• The wave theory fails to explain fundamental observations of photoelectric emission, including the dependence on frequency, the existence of a threshold frequency, and the instantaneous nature of emission.
• This highlights the limitations of the wave model in describing this phenomenon.

Einstein’s Photoelectric Equation: Energy Quantum Of Radiation

• Albert Einstein proposed the concept of light quanta, or photons, to explain the photoelectric effect. 
• According to this hypothesis, light energy is quantized into discrete units called photons, each carrying energy hν, where h is Planck's constant and ν is the frequency of light.
• Einstein's photoelectric equation 

K_max = hν − ϕ₀

• Where Kmax is the kinetic energy of emitted photoelectrons and ϕ0 is the work function of the metal. 
• This equation highlights that the kinetic energy depends on the frequency of incident radiation and the work function of the metal.
• Millikan's experiments in the early 20th century confirmed Einstein's photoelectric equation and determined the value of Planck's constant (h). 
• The slope of the V₀ versus ν graph, where V₀ is the stopping potential, is h/e, independent of the metal's nature, validating Einstein's theory and establishing the quantum nature of light.

Particle Nature Of Light: The Photon

• The photoelectric effect demonstrated that light interacts with matter as if it were composed of discrete energy packets called photons. 
• Each photon carries energy E = hf and momentum p = hf/c, where h is Planck's constant, f is the frequency of light, and c is the speed of light.
• Photons exhibit particle-like behavior, evidenced by their ability to conserve energy and momentum in collisions. 
• They are electrically neutral and remain unaffected by electric and magnetic fields. 
• Photon energy remains constant regardless of radiation intensity, with only the number of photons increasing with intensity.

Photon

Properties of Photon

• Photons travels with the speed of light.
• A photon has zero rest mass.
• Photons can not exist at rest.
• Photons travels in a straight line.
• Photons are not deviated by electric fields
• Photons are not deflected by magnetic fields.
• Photons may show diffraction under given conditions.

Wave Nature Of Matter

• Louis de Broglie proposed the hypothesis that moving particles, such as electrons, exhibit wave-like properties under certain conditions, thus introducing the concept of matter waves.
• ccording to the de Broglie relation, the wavelength (λ) associated with a particle of momentum (p) is given by λ = h / p, where h is Planck's constant. 
• This relation indicates the dual nature of matter, where particles have both particle-like and wave-like characteristics.
• The de Broglie hypothesis was experimentally confirmed through crystal diffraction experiments analogous to X-ray diffraction. 
• The experiments demonstrated that electrons indeed exhibit wave-like behavior, providing evidence for the wave nature of matter.
• The matter-wave picture introduced by de Broglie elegantly incorporates Heisenberg's uncertainty principle. 
• The principle states that it's impossible to simultaneously measure the position and momentum of a particle with absolute precision, leading to the concept of wave packets and inherent uncertainty in particle properties.

There are Some important List Of Top Physics Questions On Wave Optics Asked In CBSE CLASS XII