Quantum

Question 1

Photons

Answer 1

• Fundamental particles that make up all forms of electromagnetic radiation.
• Defined as massless “packets” or “quanta” of electromagnetic energy.
• Energy is transferred in discrete packets, not continuously.
• Each photon carries a specific amount of energy and transfers it all at once.

Question 2

Photoelectric effect

Answer 2

Photoelectric Effect:

• Definition: Electrons are emitted from a metal surface upon absorption of electromagnetic radiation.
• Photoelectrons: Electrons released from the metal.
• Key Evidence: Light is quantised (carried in discrete packets called photons).
  
  • Each electron absorbs only one photon.

Question 3

Work Function & Threshold Frequency

Answer 3

Threshold Frequency:

• The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from a metal surface.

Threshold Wavelength:

• The longest wavelength of incident electromagnetic radiation that can remove a photoelectron.

Work Function (Φ):

• The minimum energy required to release a photoelectron from a metal surface.
• Different metals have different threshold frequencies and work functions.

Examples:

• Alkali metals (e.g., sodium, potassium):
  
  • Threshold frequencies in the visible light region.
  • Weak attractive forces between electrons and metal ions.
• Transition metals (e.g., zinc, iron):
  
  • Threshold frequencies in the ultraviolet region.
  • Stronger attractive forces between electrons and metal ions.

Question 4

De broglie

Answer 4

De Broglie Equation:

• Links particle-like property (momentum) to wave-like property (wavelength), demonstrating wave-particle duality.
• Equation: λ = h/p

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Relating Kinetic Energy and Wavelength:
- Kinetic energy (E): E = (1/2)mv²
- Momentum (p): p = mv
- Combining with the de Broglie equation:
- λ = h/√(2mE)

Question 5

Wave-particle duality

Answer 5

Wave-Particle Duality of Electrons:

• Electrons exhibit wave-particle duality:
  
  • Particle nature: Have mass and charge, can be accelerated and deflected by fields.
  • Wave nature: Can diffract (e.g., at atom gaps in graphite).

…

Wave-Particle Duality of Light:

• Classical wave theory: Light behaves as a wave, shown by diffraction and interference.
• Photon model: Light also behaves as particles (photons), shown by:
  
  • Photoelectric effect: Light interacts with matter as particles.
  • Young’s Double Slit experiment: Light propagates as a wave.

Question 6

Gold-leaf electroscope

Answer 6

Gold Leaf Electroscope Experiment:

• Negatively charged zinc is attached to a central rod causing it to become negative
• A negatively charged gold leaf is repelled by a central rod.
• UV light shines on a zinc plate, emitting photoelectrons.
• Central rod loses it’s charge
• The gold leaf falls back as it loses negative charge.

Observations and Explanations:

1. UV Light Closer:
   
   • Observation: Gold leaf falls more quickly.
   • Explanation: Increased intensity emits more photoelectrons per second.
2. Higher Frequency Light:
   
   • Observation: No change in how quickly the gold leaf falls.
   • Explanation: Frequency affects kinetic energy of electrons, not emission rate.
3. Filament Light Source:
   
   • Observation: No change in gold leaf position.
   • Explanation: Frequency is below the threshold frequency; no photoelectrons emitted.
4. Positively Charged Plate:
   
   • Observation: No change in gold leaf position.
   • Explanation: Positive charge attracts emitted electrons back to the surface.
5. Instantaneous Emission:
   
   • Observation: Photoelectrons emitted immediately upon radiation exposure.
   • Explanation: A single photon interacts with a single electron; emission is instantaneous.

Question 7

Electron Diffraction Experiment

Answer 7

Electron Diffraction Experiment:

• Electrons are accelerated in an electron gun to a high potential (e.g., 5000 V) and directed through a thin graphite film.
• Graphite’s crystalline structure acts as a diffraction grating, with gaps between carbon atoms causing electron diffraction.
• A circular diffraction pattern (concentric rings) is observed on a fluorescent screen made of phosphor.

Key Observations:

• Increasing voltage:
  
  • Increases electron energy and speed.
  • Reduces the diameter of the diffraction rings.
• Decreasing voltage:
  
  • Increases the diameter of the diffraction rings.
• Wave-like behavior:
  
  • If electrons acted as particles, the screen would show a uniform distribution, not rings.

Comparison:

• Similar to the diffraction pattern produced when light passes through a diffraction grating.

Question 8

Energy levels

Answer 8

• An electron can be excited to a higher energy level by either a free electron colliding with it or by absorbing a photon of energy equal to the difference in energy.
• It will emit a wavelength at every different level it drops

Question 9

Photoelectric graph

Answer 9

Graph of KEₘₐₓ vs Frequency (f):

• Work function (Φ): y-intercept.
• Threshold frequency (f₀): x-intercept.
• Gradient: Equal to Planck’s constant (h).
• No electrons emitted below (f₀).

Photoelectric Effect Key Points:

• Threshold Frequency (f₀):
  
  • If incident photons have frequency < f₀, no electrons are emitted.
• Maximum Kinetic Energy (KEₘₐₓ):
  
  • Depends only on frequency, not intensity.
  • KE_max = hf - Φ
• Most photoelectrons have kinetic energies < KEₘₐₓ.

Question 10

Maximum Kinetic Energy & Intensity (Photoelectrons)

Answer 10

Maximum Kinetic Energy of Photoelectrons:

• Independent of the intensity of incident radiation.
• Dependent on the frequency of incident radiation.
• Each electron absorbs only one photon, so intensity (number of photons) does not affect kinetic energy.

Rate of Emission of Photoelectrons:

• Photoelectric current: Rate of photoelectron emission per second.
• Proportional to the intensity of incident radiation.
• Intensity determines the number of photons striking the metal, which increases the number of photoelectrons.

Key Point:

• Kinetic energy depends on frequency, while photoelectric current depends on intensity.