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NEB Class 12 Physics Photons Note Handwritten in PDF
Quantum Nature of Light
Before photoelectric effect, light was considered purely a wave. Wave theory predicted:
- Energy should depend on intensity (amplitude) of wave
- Any frequency of light should be able to eject electrons given enough time
- Higher intensity β more energy β more KE
But experiments completely contradicted all three predictions.
Properties of Photons
Einstein proposed light travels as discrete packets of energy called photons.
Properties of photons:
- Energy: E = hf = hc/Ξ» (h = 6.63 Γ 10β»Β³β΄ JΒ·s, Planck’s constant)
- Travel at speed of light (c = 3 Γ 10βΈ m/s) always
- Rest mass = 0, but has momentum: p = h/Ξ» = hf/c
- Not charged
- Cannot be slowed down
- One photon interacts with one electron (one-to-one interaction)
Photoelectric Effect Observations
When light falls on a metal surface:
- Electrons are emitted only if frequency f β₯ threshold frequency fβ regardless of intensity
- Below fβ: No electrons emitted no matter how intense the light
- Above fβ: Electrons emitted immediately (no time delay) even with very faint light
- KE of emitted electrons depends on frequency, not intensity
- Increasing intensity increases the number of electrons, not their KE
Work Function and Threshold Frequency
Work function (Wβ = Ο): Minimum energy needed to just remove one electron from the metal surface.
Wβ = hfβ (h Γ threshold frequency)
Each metal has a specific work function:
- Caesium: Ο β 2.0 eV (lowest used in photoelectric devices)
- Sodium: Ο β 2.3 eV
- Zinc: Ο β 4.3 eV
- Platinum: Ο β 5.6 eV (highest)
Einstein’s Photoelectric Equation
hf = Wβ + KE_max
Or: hf = hfβ + Β½mvΒ²_max
So: KE_max = h(f β fβ)
Interpretation:
- Incoming photon gives all its energy hf to the electron
- Part (Wβ) is used to escape the metal surface
- The remainder becomes kinetic energy of the photoelectron
- KE_max is a straight line graph vs frequency f slope = h (Planck’s constant)
Stopping Potential
When a retarding voltage is applied to stop all photoelectrons:
eVβ = KE_max = h(f β fβ)
Vβ = h(f β fβ)/e
- Stopping potential Vβ increases linearly with frequency
- Graph of Vβ vs f β straight line, slope = h/e β from this, h can be calculated
- This is exactly what Millikan did to verify Einstein’s equation and measure h
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Frequently Asked Questions
What is photoelectric effect in Class 12 Physics?
Photoelectric effect is emission of electrons from a metal surface when light of sufficient frequency falls on it. Key observations: threshold frequency exists below which no emission occurs regardless of intensity. KE of emitted electrons depends on frequency not intensity. Emission is instantaneous. Wave theory failed to explain all three observations.
What is Einstein’s photoelectric equation Class 12?
Einstein’s equation: hf = Wβ + Β½mvΒ²_max = hfβ + KE_max. Incoming photon gives all energy hf to one electron. Part Wβ is used to escape the metal surface. Remainder becomes kinetic energy of photoelectron. KE_max = h(fβfβ) increases linearly with frequency proven experimentally by Millikan to calculate Planck’s constant.
What is work function and threshold frequency Class 12?
Work function (Wβ = hfβ): minimum energy to remove one electron from metal surface. Threshold frequency (fβ): minimum frequency of light that can cause photoelectric emission. Below fβ no emission occurs regardless of light intensity. Different metals have different work functions: caesium (2.0 eV, lowest) to platinum (5.6 eV, highest).
What is stopping potential in photoelectric effect?
Stopping potential (Vβ): minimum negative voltage that stops all photoelectrons. At stopping potential: eVβ = KE_max = h(fβfβ). Graph of Vβ vs frequency gives straight line with slope h/e = 4.14Γ10β»ΒΉβ΅ VΒ·s. From this Planck’s constant h is calculated. Millikan used this to verify Einstein’s equation experimentally.
Why did wave theory fail to explain photoelectric effect?
Wave theory predicted: KE should increase with intensity experimentally wrong, only frequency affects KE. There should be a time delay with faint light experimentally emission is instantaneous. Threshold frequency should not exist experimentally it does. Einstein’s photon theory correctly explains all three observations.
What are properties of photons in Class 12 Physics?
Photons: energy E = hf = hc/Ξ». Momentum p = h/Ξ» = hf/c. Rest mass = zero. Travel at speed of light always. No charge. Cannot be slowed down. One photon transfers all its energy to one electron (one-to-one interaction). Intensity determines number of photons per second not energy per photon which depends on frequency alone.
What is the difference between photoelectric effect and thermionic emission?
Photoelectric effect: electrons emitted when light of sufficient frequency falls on metal requires photons above threshold frequency. Thermionic emission: electrons emitted when metal is heated to high temperature requires thermal energy. Both require electrons to gain enough energy to overcome the work function barrier.
Which photoelectric effect topics are most important for Exam?
Most important: Einstein’s photoelectric equation derivation and explanation (4 marks), stopping potential and its relation to frequency (4 marks), work function and threshold frequency (2 marks), failure of wave theory (2 marks). Einstein’s equation numericals calculating KE, stopping potential, or Planck’s constant appear in every NEB board paper.
