NEET Physics — Modern Physics Complete Chapter Guide

Chapter Overview & Weightage

Modern Physics covers the photoelectric effect, Bohr’s atomic model, nuclear physics, radioactivity, and semiconductors. NEET asks 4-5 questions, heavily tilted towards photoelectric effect and Bohr model numericals.

Modern Physics carries 8-10% weightage in NEET with 4-5 questions. Photoelectric effect and Bohr model calculations appear every year. Nuclear reactions and semiconductor basics contribute 1-2 additional questions.

Key Concepts You Must Know

Tier 1 (Core)

Photoelectric effect: $KE_{max} = h\nu - \phi$ (Einstein’s equation), threshold frequency, stopping potential
Bohr model: $r_n = 0.53n^2/Z$ Angstrom, $E_n = -13.6Z^2/n^2$ eV, $v_n \propto Z/n$
de Broglie wavelength: $\lambda = h/p = h/mv$
Radioactive decay: $N = N_0 e^{-\lambda t}$ , half-life $t_{1/2} = 0.693/\lambda$
Mass-energy equivalence: $E = mc^2$ , mass defect, binding energy

Tier 2 (Frequently tested)

Hydrogen spectrum: Lyman (UV, $n \to 1$ ), Balmer (visible, $n \to 2$ ), Paschen (IR, $n \to 3$ )
Nuclear reactions: alpha decay ( $Z-2, A-4$ ), beta decay ( $Z+1, A$ same), gamma (no change)
Binding energy per nucleon curve — most stable around $A = 56$ (Fe)
p-n junction diode: forward bias, reverse bias, depletion region, barrier potential

Tier 3 (Occasionally tested)

Nuclear fission and fusion basics
Logic gates: AND, OR, NOT, NAND, NOR
Zener diode as voltage regulator

Important Formulas

KE_{max} = h\nu - \phi = eV_0

Where:

$h\nu$ = energy of incident photon
$\phi = h\nu_0$ = work function (minimum energy to eject electron)
$V_0$ = stopping potential
$\nu_0$ = threshold frequency

Key facts: KE increases linearly with frequency (above threshold). Increasing intensity increases photocurrent but NOT KE.

r_n = \frac{0.53 n^2}{Z} \text{ Angstrom}

E_n = \frac{-13.6 Z^2}{n^2} \text{ eV}

v_n = \frac{2.18 \times 10^6 \times Z}{n} \text{ m/s}

Transition energy: $\Delta E = 13.6Z^2\left(\frac{1}{n_1^2} - \frac{1}{n_2^2}\right)$ eV

N = N_0 e^{-\lambda t} = N_0 \left(\frac{1}{2}\right)^{t/t_{1/2}}

t_{1/2} = \frac{0.693}{\lambda}

\text{Activity } A = \lambda N = A_0 e^{-\lambda t}

After $n$ half-lives: $N = N_0/2^n$

For Bohr model NEET problems, most calculations involve the energy formula $E_n = -13.6/n^2$ eV for hydrogen. The transition from $n = 3$ to $n = 2$ gives the first line of Balmer series. From $n = 2$ to $n = 1$ gives the first line of Lyman series. Know these specific transitions.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: The work function of a metal is 2 eV. Find the maximum kinetic energy of photoelectrons when light of energy 5 eV falls on it.

Solution:

KE_{max} = h\nu - \phi = 5 - 2 = \mathbf{3 \text{ eV}}

PYQ 2 — NEET 2023

Problem: The

energy of an electron in the 2nd orbit of hydrogen atom is:

Solution:

E_2 = \frac{-13.6}{2^2} = \frac{-13.6}{4} = \mathbf{-3.4 \text{ eV}}

PYQ 3 — NEET 2022

Problem: The half-life of a radioactive substance is 20 days. What fraction remains after 60 days?

Solution:

Number of half-lives: $n = 60/20 = 3$

Fraction remaining: $\left(\frac{1}{2}\right)^3 = \mathbf{1/8}$

Expert Strategy

Week 1: Photoelectric effect — Einstein’s equation problems, stopping potential, threshold frequency. This is the most predictable topic in NEET physics.

Week 2: Bohr model — energy levels, transitions, spectral series. Practice calculating wavelength/frequency of emitted photon for specific transitions.

Week 3: Radioactivity and semiconductors. Half-life calculations are straightforward. For semiconductors, know p-n junction behaviour in forward and reverse bias.

Common Traps

Trap 1 — Increasing intensity increases photocurrent, NOT kinetic energy. Intensity = more photons = more electrons ejected = more current. But each photon’s energy depends on frequency, not intensity. KE depends only on frequency.

Trap 2 — Energy levels are negative. $E_n = -13.6/n^2$ eV. The ground state ( $n = 1$ ) has the MOST negative energy (most tightly bound). $n = \infty$ has $E = 0$ (free electron). Ionisation energy = $|E_1| = 13.6$ eV for hydrogen.

Trap 3 — In beta decay, a neutron converts to proton + electron. The mass number stays the same but atomic number increases by 1. In alpha decay, both $Z$ and $A$ decrease ( $Z - 2$ , $A - 4$ ).

Trap 4 — Half-life problems: after $n$ half-lives, fraction remaining is $(1/2)^n$ , not $n/2$ . After 1 half-life: 1/2. After 2: 1/4. After 3: 1/8. The decay is exponential, not linear.