NDA Physics · Teaching notes

Modern Physics — NDA Physics

Modern Physics is one of NDA Physics's most reliable scoring chapters — about 25 PYQs across 2017–2025, every single one EASY or MODERATE, with ZERO hard questions. The chapter is overwhelmingly recall: named experiments, named scientists, fixed facts (M-shell holds 18 electrons, hydrogen ionisation energy is 13.6 eV, the cochlea... no, the cathode... of the atom), acronyms, and a handful of one-step formula plug-ins. It teaches in six progressive movements: (1) Photoelectric effect — light ejecting electrons, Einstein's photon picture, and the E = hf / cutoff-wavelength one-liners; (2) Atomic structure — cathode rays, Rutherford's nucleus, Bohr's stable orbits, shell electron capacities, and ionisation energy; (3) Nuclear physics — fission vs fusion, the reactor, fuel minerals, and how radioactivity is measured; (4) Quantum and modern EM — X-rays, semiconductors (p-type/n-type), and the Raman effect; (5) Scientists and their discoveries — the match-the-pair recall table; (6) Scientific acronyms — LED, LASER, LIGO and friends. Drill the reference tables, learn the four one-step formulas, and this chapter is near-free marks.

NDA Physics strategy NDA home

Subtopic notes

PYQ weightage by concept

19 concepts · 25 PYQs — where the marks actually sit, so you know what to drill first

Photoelectric Effect: Light as Particles4 PYQs · 16%

Concept	PYQs	Share
The photon — light carries energy in discrete packets E = hf	1	4%
Photoelectric emission — light ejecting electrons from a metal	1	4%
Who explained the photoelectric effect — Einstein and the Nobel Prize	1	4%
Cutoff wavelength and the energy-voltage link	1	4%

Atomic Structure: Models, Shells, and Energy6 PYQs · 24%

Concept	PYQs	Share
Electron shells and ionisation energy — the fixed numbers	3	12%
Cathode rays — the discovery of the electron	1	4%
Rutherford's alpha-scattering — the nucleus	1	4%
Bohr's model — electrons in stable orbits without radiating	1	4%

Nuclear Physics: Fission, Fusion, and Reactors5 PYQs · 20%

Concept	PYQs	Share
The nuclear reactor — controlled fission and its parts	2	8%
Nuclear fuel, measuring radioactivity, and radiation types	2	8%
Nuclear energy — fission, fusion, and E = mc²	1	4%

Quantum and Modern EM: X-rays, Semiconductors, Scattering5 PYQs · 20%

Concept	PYQs	Share
Semiconductors — p-type, n-type, and the I-V graph	2	8%
X-rays — properties and uses	1	4%
Dimensions of Planck's constant — same as angular momentum	1	4%
Scattering phenomena — the Raman effect	1	4%

Scientists and Discoveries: Match the Pair3 PYQs · 12%

Concept	PYQs	Share
LIGO — confirming gravitational waves and general relativity	2	8%
Key modern-physics discoveries and their scientists	1	4%

Scientific Acronyms: Full Forms to Memorise2 PYQs · 8%

Concept	PYQs	Share
Device and optics acronyms — LED, LASER, LCD	1	4%
Big-science and research acronyms — LIGO, LASER, MASER	1	4%

Formula & revision sheet

5 formulas · 13 reference tables · 23 gotchas across all subtopics — the exam-eve cheat-sheet

Photoelectric Effect: Light as Particles

Formulas (2)

The photon — light carries energy in discrete packets E = hf · Energy of a photon $E = hf = \dfrac{hc}{\lambda}$
Cutoff wavelength and the energy-voltage link · Cutoff wavelength of X-rays $\lambda_{min} = \dfrac{hc}{eV}$

Reference tables (1)

Who explained the photoelectric effect — Einstein and the Nobel Prize3 rows

Person / idea	Contribution
Albert Einstein	Explained the photoelectric effect using the photon/quantum idea (1905) NDA 2019 — the photoelectric effect was explained by Albert Einstein (not Bohr, Planck, or Rutherford).
Max Planck	Introduced energy quanta $E = hf$ (Planck's constant); the quantum seed Einstein used
Heinrich Hertz	First OBSERVED the photoelectric effect experimentally (but did not explain it)

Distinguish the OBSERVER (Hertz) from the EXPLAINER (Einstein). The exam asks who explained it — that is Einstein.

Watch out for (6)

Brightness does NOT change photon energy
→ The photon — light carries energy in discrete packets E = hf
Energy grows with frequency, falls with wavelength
→ The photon — light carries energy in discrete packets E = hf
Threshold is about FREQUENCY, not intensity
→ Photoelectric emission — light ejecting electrons from a metal
Einstein's Nobel was for the photoelectric effect, NOT relativity
→ Who explained the photoelectric effect — Einstein and the Nobel Prize
Observed vs explained
→ Who explained the photoelectric effect — Einstein and the Nobel Prize
Cutoff wavelength is inversely related to voltage
→ Cutoff wavelength and the energy-voltage link

Atomic Structure: Models, Shells, and Energy

Formulas (1)

Bohr's model — electrons in stable orbits without radiating · Bohr energy levels of hydrogen $E_n = -\dfrac{13.6}{n^2}\ \text{eV}$

Reference tables (3)

Cathode rays — the discovery of the electron4 rows

Property of cathode rays	Correct statement
What they are	A stream of electronsQ
Direction of travel	From cathode to anode (negative to positive) NDA 2019 — the FALSE statement was "cathode ray particles start from the anode and move towards the cathode." They go cathode to anode.
Charge	Negative
Path	Straight line; deflected by electric and magnetic fields

The most common trap reverses the direction: cathode rays start at the CATHODE, not the anode.

Rutherford's alpha-scattering — the nucleus3 rows

Observation	Conclusion
Most alpha particles pass straight through	Atom is mostly empty space
A few alpha particles deflect at large angles / rebound	A tiny, dense, positively charged nucleus exists NDA 2021 — Rutherford's alpha-scattering experiment discovered the atomic NUCLEUS.
Almost all mass concentrated centrally	Nucleus holds the protons (and neutrons)

Rutherford = nucleus. Do not confuse with Chadwick (neutron) or Thomson (electron).

Electron shells and ionisation energy — the fixed numbers6 rows

Quantity	Value	How to get it
K-shell (n=1) capacity	2 electrons	$2n^2 = 2(1)^2 = 2$
L-shell (n=2) capacity	8 electrons	$2n^2 = 2(2)^2 = 8$
M-shell (n=3) capacity	18 electrons	$2n^2 = 2(3)^2 = 18$ NDA 2021 — the M-shell holds a maximum of 18 electrons.
N-shell (n=4) capacity	32 electrons	$2n^2 = 2(4)^2 = 32$
Hydrogen ionisation energy	13.6 eV	Depth of the n=1 ground stateQ
Energy in atomic bonds	Chemical energy	Stored in the links between atomsQ

Shell capacity is 2n². Hydrogen ionisation energy 13.6 eV is the single most-repeated number in the chapter.

Watch out for (5)

Cathode to anode, never anode to cathode
→ Cathode rays — the discovery of the electron
Rutherford found the nucleus, not the neutron
→ Rutherford's alpha-scattering — the nucleus
Bohr's electrons radiate only when they JUMP
→ Bohr's model — electrons in stable orbits without radiating
Shell capacity is 2n², not a fixed 8
→ Electron shells and ionisation energy — the fixed numbers
Hydrogen ionisation energy = 13.6 eV (positive energy IN)
→ Electron shells and ionisation energy — the fixed numbers

Nuclear Physics: Fission, Fusion, and Reactors

Formulas (1)

Nuclear energy — fission, fusion, and E = mc² · Mass-energy equivalence $E = mc^2$

Reference tables (2)

The nuclear reactor — controlled fission and its parts5 rows

Reactor component	Role
Fuel (uranium-235)	Undergoes fission to release energy
Moderator (graphite, heavy water)	Slows down fast neutrons
Control rods (cadmium, boron)	Absorb neutrons to control the reaction rate
Coolant	Removes heat from the core
A mechanism to reduce CO₂ emission	Does NOT belong to a reactor NDA 2024 — the item that does NOT belong to a nuclear reactor is "a mechanism to reduce CO₂ emission" (reactors emit no CO₂ in the first place).

Principle = controlled fission. The odd-one-out trap is the CO₂-reduction mechanism — nuclear power produces no CO₂, so no such part exists.

Nuclear fuel, measuring radioactivity, and radiation types3 rows

Radiation	What it is	Stopped by
Alpha (α)	Helium nucleus (2 protons + 2 neutrons), charge +2	A sheet of paper Most ionising, least penetrating.
Beta (β)	Fast electron, charge -1	A few mm of aluminium
Gamma (γ)	High-energy electromagnetic wave, no charge	Thick lead or concrete Least ionising, most penetrating.

Penetration: γ > β > α. Ionising power: α > β > γ. Alpha is stopped by paper; gamma needs thick lead.

Watch out for (4)

Fission splits, fusion joins — and reactors use fission
→ Nuclear energy — fission, fusion, and E = mc²
Reactor = CONTROLLED fission (a bomb is uncontrolled)
→ The nuclear reactor — controlled fission and its parts
Penetration and ionising power run OPPOSITE ways
→ Nuclear fuel, measuring radioactivity, and radiation types
Pitchblende, not coal or limestone
→ Nuclear fuel, measuring radioactivity, and radiation types

Quantum and Modern EM: X-rays, Semiconductors, Scattering

Formulas (1)

Dimensions of Planck's constant — same as angular momentum · Dimensions of Planck's constant $[h] = \dfrac{[E]}{[f]} = \text{J}\cdot\text{s} = [\text{angular momentum}]$

Reference tables (3)

X-rays — properties and uses4 rows

Statement about X-rays	True or false
Wavelength about 1 Å	True
Generated by bombarding a metal target with high-energy electrons	True
Used for radar systems because of their short wavelength	FALSE NDA 2023 — the false statement is that X-rays are used for radar. Radar uses long radio/microwaves, not X-rays.
Used to treat certain cancers	True

Everything about X-rays is true EXCEPT the radar claim. Radar = radio/microwave, not X-ray.

Semiconductors — p-type, n-type, and the I-V graph2 rows

Type	Majority charge carriers	Created by doping with
p-type	Holes (positive)	Trivalent impurity (e.g. boron)Q NDA 2017 — the majority charge carriers in a p-type semiconductor are holes.
n-type	Electrons (negative)	Pentavalent impurity (e.g. phosphorus)

p-type = holes (positive). n-type = electrons (negative). A plain wafer's I-V graph is a straight line through the origin.

Scattering phenomena — the Raman effect4 rows

Phenomenon	What happens
Raman effect	Scattered light's frequency CHANGES (inelastic scattering) NDA 2021 — light scattered with a changed frequency is the Raman effect.
Rayleigh scattering	Scattering with NO frequency change (elastic)
Photoelectric effect	Light ejects electrons from a metal — not scattering
Rutherford scattering	Alpha particles scatter off atomic nuclei

Frequency CHANGES = Raman. No change = Rayleigh. The other two are different phenomena entirely.

Watch out for (4)

X-rays for radar is the WRONG statement
→ X-rays — properties and uses
h is angular momentum, not linear momentum or torque
→ Dimensions of Planck's constant — same as angular momentum
p-type carriers are HOLES (positive), n-type are electrons
→ Semiconductors — p-type, n-type, and the I-V graph
Raman = frequency CHANGES; Rayleigh = no change
→ Scattering phenomena — the Raman effect

Scientists and Discoveries: Match the Pair

Reference tables (2)

Key modern-physics discoveries and their scientists6 rows

Scientist	Discovery / contribution
James Chadwick	Neutron
Albert Einstein	Photoelectric effect explanation; relativity (E = mc²)
Marie Curie	Radium (and polonium); radioactivity NDA 2021 — in the match list, only "Marie Curie : Radium" was correctly matched (Chadwick was wrongly paired with photoelectric effect, Einstein with neutron).
Ernest Rutherford	Atomic nucleus (alpha scattering)
J. J. Thomson	Electron (cathode rays)
Niels Bohr	Stable electron orbits (Bohr model)

The exam plants swaps: Chadwick is the NEUTRON (not photoelectric), Einstein is the PHOTOELECTRIC EFFECT (not the neutron), Curie is RADIUM.

LIGO — confirming gravitational waves and general relativity3 rows

Item	Fact
LIGO full form	Laser Interferometer Gravitational-wave Observatory
LIGO confirmed	Gravitational waves predicted by Einstein's General Theory of Relativity NDA 2024 — the LIGO experiment confirmed a prediction of the General Theory of Relativity.
LED full form	Light Emitting Diode (a semiconductor device)Q

LIGO detected gravitational waves and so confirmed General Relativity. LED is a Light Emitting Diode.

Watch out for (2)

Chadwick = neutron, Einstein = photoelectric effect
→ Key modern-physics discoveries and their scientists
LIGO confirmed GENERAL relativity (via gravitational waves)
→ LIGO — confirming gravitational waves and general relativity

Scientific Acronyms: Full Forms to Memorise

Reference tables (2)

Device and optics acronyms — LED, LASER, LCD4 rows

Acronym	Full form
LED	Light Emitting Diode NDA 2018 / 2021 — LED stands for Light Emitting Diode.
LASER	Light Amplification by Stimulated Emission of Radiation
LCD	Liquid Crystal Display
CFL	Compact Fluorescent Lamp

LED = Light Emitting Diode. Note LASER's expansion turns on "Stimulated Emission".

Big-science and research acronyms — LIGO, LASER, MASER4 rows

Acronym	Full form
LIGO	Laser Interferometer Gravitational-wave Observatory NDA 2019 — LIGO stands for Laser Interferometer Gravitational-wave Observatory.
MASER	Microwave Amplification by Stimulated Emission of Radiation
SONAR	Sound Navigation and Ranging
RADAR	Radio Detection and Ranging

LIGO opens with LASER and ends with OBSERVATORY. RADAR uses radio waves; SONAR uses sound.

Watch out for (2)

LED's D is DIODE, not Display
→ Device and optics acronyms — LED, LASER, LCD
LIGO starts with LASER, not Light
→ Big-science and research acronyms — LIGO, LASER, MASER