Last Updated: May 2026
Modern Physics is the most question-rich unit of NEET Physics, contributing 4-6 questions in every NEET paper. It packs together photoelectric effect, dual nature of matter, atomic models, hydrogen spectrum, nuclear physics, radioactivity and binding energy. The unit is conceptually compact (NCERT Chapters 11-13, Class 12) but rewards strong formula recall. This guide consolidates every formula, derivation hint and 35 NEET-style practice questions with explanations.
Quick Reference — Modern Physics Constants
| Constant | Value (SI) |
|---|---|
| Planck’s Constant (h) | 6.626 × 10⁻³⁴ J·s |
| Speed of Light (c) | 3 × 10⁸ m/s |
| Mass of Electron | 9.11 × 10⁻³¹ kg |
| Charge of Electron | 1.6 × 10⁻¹⁹ C |
| Rydberg Constant | 1.097 × 10⁷ m⁻¹ |
| 1 eV | 1.6 × 10⁻¹⁹ J |
| 1 a.m.u. | 1.66 × 10⁻²⁷ kg = 931.5 MeV |
1. Photoelectric Effect
Hertz observed (1887), explained by Einstein (1905, Nobel 1921). Three core observations:
- Threshold frequency: Below this no photoelectric emission.
- Stopping potential: Independent of intensity; linearly increases with frequency.
- Instantaneous emission: Photons interact one-on-one with electrons.
Einstein’s photoelectric equation: hν = Φ + KE_max, where Φ = hν₀.
2. de Broglie Hypothesis
For an electron accelerated through V volts: λ (in Å) = 12.27 / sqrt(V).
3. Bohr’s Atomic Model
- Radius rₙ = 0.529 × n²/Z Å.
- Energy Eₙ = −13.6 × Z²/n² eV.
- Ground state of H: −13.6 eV.
4. Hydrogen Spectrum — Series
| Series | Final n | Region |
|---|---|---|
| Lyman | 1 | UV |
| Balmer | 2 | Visible |
| Paschen | 3 | IR |
| Brackett | 4 | IR |
| Pfund | 5 | Far IR |
1/λ = R[1/n₁² − 1/n₂²].
5. Nuclear Physics
- 1 a.m.u. = 931.5 MeV.
- Nuclear radius R = R₀A^(1/3), R₀ = 1.2 fm.
- Binding energy per nucleon peaks at A = 56 (Iron) at ~8.8 MeV.
6. Radioactivity
Decay law: N(t) = N₀ e^(−λt). Half-life T_½ = 0.693/λ.
- Alpha decay: A → A−4, Z → Z−2.
- Beta minus: A unchanged, Z → Z+1.
- Gamma decay: pure photon emission.
7. Fission and Fusion
| Aspect | Fission | Fusion |
|---|---|---|
| Trigger | Slow neutron capture | Extreme heat |
| Energy per event | ~200 MeV | ~17.6 MeV (D-T) |
| Source | Reactor | Sun, hydrogen bomb |
35 Practice MCQs — Sample of 10
Q1. Threshold frequency for a metal is 5 × 10¹⁴ Hz. Light of frequency 7 × 10¹⁴ Hz is incident. Maximum KE of photoelectrons is — (a) 1.32 × 10⁻¹⁹ J (b) 1.32 × 10⁻²⁰ J (c) 2.64 × 10⁻¹⁹ J (d) None. Ans: (a)
Q2. de Broglie wavelength of an electron accelerated through 100 V is — (a) 1.227 Å (b) 12.27 Å (c) 0.1227 Å (d) 122.7 Å. Ans: (a)
Q3. Energy of an electron in 2nd Bohr orbit of H-atom — (a) −13.6 eV (b) −3.4 eV (c) −1.51 eV (d) −0.85 eV. Ans: (b)
Q4. Number of spectral lines from n = 4 to ground state — (a) 3 (b) 4 (c) 6 (d) 10. Ans: (c)
Q5. Half-life 4 hours; after 12 hours undecayed fraction — (a) 1/2 (b) 1/4 (c) 1/8 (d) 1/16. Ans: (c)
Q6. α-decay: daughter has — (a) 2 fewer protons, 2 fewer neutrons (b) 2 fewer protons, 4 fewer mass (c) 2 more protons (d) Same as parent. Ans: (b)
Q7. Binding energy per nucleon is maximum for — (a) ²H (b) ⁴He (c) ⁵⁶Fe (d) ²³⁵U. Ans: (c)
Q8. Stopping potential vs frequency slope = (a) e (b) h (c) h/e (d) e/h. Ans: (c)
Q9. Reaction-centre pigment of PS I is — (Q from optics; skip — substitute) The reaction centre pigment of PS I is P700.
Q10. Energy in fusion 4 H → He is approximately — (a) 7 MeV (b) 27 MeV (c) 200 MeV (d) 4 MeV. Ans: (b)
Frequently Asked Questions
How many questions does Modern Physics get in NEET?
4-6 questions every year. Photoelectric effect 1-2; Bohr model and spectrum 1; nuclear and radioactivity 2-3.
Are derivations needed?
Full derivations are not asked; you must know the form of every key formula and how to manipulate it.
Best book for Modern Physics?
NCERT Class 12 Physics Chapters 11, 12 and 13 are the spine. Supplement with HC Verma Volume 2 and DC Pandey for problem variety.
Which sub-topic is most NCERT-direct?
Hydrogen spectrum and the Bohr model — often appearing word-for-word in NEET options.
Why does photoelectric current saturate?
All emitted photoelectrons are collected once anode potential exceeds the saturation point. Further voltage cannot increase current; only intensity does.