Last Updated: May 2026
JEE Main Modern Physics 2027 contributes 5–7 questions every JEE Main paper — making it the highest-yield Physics chapter group. Modern Physics covers Photoelectric Effect, Atomic Models, Nuclei, and Semiconductor Electronics. This guide gives compact NCERT-aligned notes plus 40 practice problems.
Why Modern Physics is JEE-Critical
Modern Physics combines conceptual clarity with formula-application — so one well-prepared chapter cluster easily yields 20+ marks. The topics are Class 12 Chapters 11–14, and JEE testers consistently include 1–2 from each.
1. Photoelectric Effect
When light of frequency ν falls on a metal, electrons (called photoelectrons) are ejected if ν ≥ ν0 (threshold frequency).
Einstein’s Photoelectric Equation
Kmax = hν − φ0 = hν − hν0
Where φ0 = work function = hν0.
Key Observations
- Kmax depends only on ν (not on intensity)
- Number of photoelectrons depends on intensity
- Below threshold ν0, no photoelectrons (regardless of intensity)
- Stopping potential V0: eV0 = Kmax
Worked Example
Light of wavelength 400 nm falls on a metal of work function 2.0 eV. Find Kmax.
E = hc/λ = 1240/400 eV·nm/nm = 3.1 eV. Kmax = 3.1 − 2.0 = 1.1 eV.
2. de Broglie Wavelength
λ = h/p = h/(mv) = h/√(2mK)
For a charged particle accelerated through V volts: λ = h/√(2mqV).
Electron specific: λ (in Å) = 12.27/√V (V in volts)
3. Atomic Models
3.1 Bohr’s Model — Hydrogen Atom
| Quantity | Formula |
|---|---|
| Radius | rn = 0.529 × n²/Z Å |
| Velocity | vn = (Zc)/(137n) = 2.18×10⁶ × Z/n m/s |
| Energy | En = −13.6 × Z²/n² eV |
| Frequency | ν = (E2−E1)/h |
| Angular momentum | L = nh/(2π) |
3.2 Spectral Series of Hydrogen
| Series | Region | Transition (n1 →) |
|---|---|---|
| Lyman | UV | 1 |
| Balmer | Visible | 2 |
| Paschen | IR | 3 |
| Brackett | IR | 4 |
| Pfund | IR | 5 |
Wavenumber: 1/λ = R(1/n1² − 1/n2²), R = 1.097 × 10⁷ m⁻¹ (Rydberg constant)
4. Nuclei
4.1 Mass-Energy Equivalence
E = mc². 1 atomic mass unit (amu) = 931.5 MeV.
4.2 Binding Energy and Mass Defect
Δm = [Z mp + N mn] − Mnucleus
Binding Energy (BE) = Δm × c²
BE per nucleon peaks at ~A=56 (iron region) and falls for both lighter (fusion) and heavier (fission) nuclei.
4.3 Radioactive Decay
N(t) = N0 e−λt
Half-life: T1/2 = 0.693/λ = ln 2 / λ
Mean life: τ = 1/λ
4.4 Types of Decay
| Decay | Particle Emitted | Daughter Z, A |
|---|---|---|
| α | ⁴He nucleus | Z−2, A−4 |
| β⁻ | e⁻, antineutrino | Z+1, A |
| β⁺ | e⁺, neutrino | Z−1, A |
| γ | Photon | Z, A unchanged |
5. Semiconductor Electronics
5.1 Intrinsic vs Extrinsic Semiconductors
- Intrinsic: pure Si or Ge; ne = nh
- n-type: doped with pentavalent (P, As, Sb); majority carriers = electrons
- p-type: doped with trivalent (B, Al, Ga, In); majority carriers = holes
5.2 p-n Junction Diode
- Forward bias: low resistance, current flows
- Reverse bias: high resistance, negligible current (until breakdown)
- Knee/cut-in voltage: 0.3 V (Ge), 0.7 V (Si)
5.3 Logic Gates (Truth Tables)
| A | B | AND | OR | NAND | NOR | XOR |
|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 | 0 | 0 | 0 |
JEE Main High-Yield Quick Facts
- 1 eV = 1.6 × 10⁻¹⁹ J; hc = 1240 eV·nm (memorise!)
- Bohr radius = 0.529 Å for hydrogen ground state
- Rydberg constant R = 1.097 × 10⁷ m⁻¹
- Ground state energy of hydrogen = −13.6 eV
- Maximum BE per nucleon at A ~ 56 (Fe)
- NAND and NOR are universal gates — can build any logic circuit
40 Practice MCQs
[cg_quiz id=”jee-modern-physics-2027″]
FAQ
Q1. How many questions from Modern Physics in JEE Main?
5–7 questions consistently. JEE Main 2024 had 6 (2 photoelectric, 1 Bohr, 1 nuclei, 2 semiconductor).
Q2. Is the photoelectric effect a difficult chapter?
No — it’s formula-driven. Master Einstein’s equation + stopping potential and you’ll get every question.
Q3. Are spectral series important?
Yes — JEE testers love asking which series falls in visible/UV/IR. Memorise the table.
Q4. Should I derive Bohr’s formulas or memorise?
Memorise. Derivations are not asked in JEE Main. Save time for problem-solving.
Q5. Logic gates — JEE Main or just for boards?
Both. JEE Main occasionally asks truth-table interpretation or universal-gate questions.