NDA Physics · Free 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.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Photoelectric Effect: Light as Particles
4 PYQsWhen light of high enough frequency strikes a metal surface, it ejects electrons instantly; the energy of each ejected electron depends on the light's frequency (colour), not its brightness.
Atomic Structure: Models, Shells, and Energy
6 PYQsThe atom is a tiny dense nucleus (protons + neutrons) surrounded by electrons in fixed energy shells; a sequence of experiments — cathode rays, Rutherford scattering, Bohr's orbits — revealed this picture.
Nuclear Physics: Fission, Fusion, and Reactors
5 PYQsThe nucleus stores enormous energy released by fission (splitting heavy nuclei) or fusion (joining light nuclei); reactors use controlled fission, and Einstein's E = mc² ties the released energy to lost mass.
Quantum and Modern EM: X-rays, Semiconductors, Scattering
5 PYQsA grab-bag of modern applications: X-rays (high-energy EM made by stopping fast electrons), semiconductors (p-type and n-type materials that follow Ohm's law), and the Raman effect (light changing frequency when scattered).
Scientists and Discoveries: Match the Pair
3 PYQsA pure-recall table of who discovered or explained what in modern physics — Chadwick (neutron), Einstein (photoelectric effect, relativity), Marie Curie (radium), and the LIGO confirmation of gravitational waves.
Scientific Acronyms: Full Forms to Memorise
2 PYQsA short list of physics-and-technology acronyms whose full forms the NDA tests directly — LED, LASER, LIGO and a few common companions.
Formula & revision sheet
5 formulas · 13 reference tables · 23 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
5 formulas · 13 reference tables · 23 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
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 (Planck's constant); the quantum seed Einstein used |
| Heinrich Hertz | First OBSERVED the photoelectric effect experimentally (but did not explain it) |
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
Formulas (1)
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 |
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) |
Electron shells and ionisation energy — the fixed numbers6 rows
| Quantity | Value | How to get it |
|---|---|---|
| K-shell (n=1) capacity | 2 electrons | |
| L-shell (n=2) capacity | 8 electrons | |
| M-shell (n=3) capacity | 18 electrons | NDA 2021 — the M-shell holds a maximum of 18 electrons. |
| N-shell (n=4) capacity | 32 electrons | |
| 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 |
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
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). |
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. |
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
Formulas (1)
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 |
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) |
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 |
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
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) |
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 |
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
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 |
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 |
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
PYQ weightage by concept
19 concepts · 25 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
19 concepts · 25 PYQs — where the marks actually sit, so you know what to drill first
| 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% |
| 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% |
| 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% |
| 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% |
| Concept | PYQs | Share |
|---|---|---|
| LIGO — confirming gravitational waves and general relativity | 2 | 8% |
| Key modern-physics discoveries and their scientists | 1 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Device and optics acronyms — LED, LASER, LCD | 1 | 4% |
| Big-science and research acronyms — LIGO, LASER, MASER | 1 | 4% |
Test yourself on Modern Physics
10 past NDA questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.