Physics
🔒 Log in to trackNuclear physics, inventions and scientists
🔒 Log in to trackThe atom has a nucleus of protons and neutrons with electrons around it. Radioactivity, fission and fusion come from the nucleus. Exams ask who found what, which invention belongs to whom, and India's nuclear and space firsts.
Inside the atom
An atom has a tiny, dense nucleus of protons and neutrons. Electrons move around it.
- Electron: negative charge. Found by J.J. Thomson (1897) in cathode-ray experiments.
- Proton: positive charge. Seen in canal rays by E. Goldstein.
- Neutron: no charge. Discovered by James Chadwick (1932).
- Nucleus: shown by Rutherford's gold-foil experiment (1911). Most alpha particles went straight through and a few bounced back. So nearly all the mass and positive charge sit in a tiny centre.
- Bohr (1913): added fixed orbits for electrons.
Radioactivity
Henri Becquerel (1896) found that uranium salts give out invisible, penetrating rays. This is radioactivity. It belongs to heavy, unstable nuclei.
Marie and Pierre Curie discovered polonium and radium. Marie Curie won Nobel Prizes in Physics (1903) and Chemistry (1911).
| Ray | What it is | Penetration |
|---|---|---|
| Alpha | Helium nucleus | Least. Paper stops it. |
| Beta | Fast electron | Medium. Aluminium stops it. |
| Gamma | Electromagnetic wave | Most. Thick lead needed. |
A Geiger–Müller counter detects radiation. Willard Libby's carbon-14 dating finds the age of old organic remains.
Fission, fusion and reactors
- Fission: a heavy nucleus such as U-235 splits when a neutron hits it. It releases energy and more neutrons. That is a chain reaction. Uncontrolled, it is the atom bomb (Hiroshima, 1945). Controlled, it makes electricity.
- Fusion: light nuclei (hydrogen isotopes) join into a heavier one. It powers the Sun and stars, and the hydrogen bomb.
- Both follow Einstein's . A little lost mass becomes a lot of energy.
Rule: Fission means split. Fusion means join. Reactors use fission, not fusion.
In a reactor, the moderator (heavy water, graphite) slows neutrons. Control rods (cadmium, boron) absorb neutrons. A coolant carries the heat away.
India's atomic and space firsts
Homi J. Bhabha is the father of India's nuclear programme. He set up the Tata Institute of Fundamental Research (1945) and BARC at Trombay.
| Item | Fact |
|---|---|
| Apsara (1956) | First research reactor in Asia |
| Tarapur (1969) | First nuclear power station, Maharashtra |
| Pokhran-I (1974) | First nuclear test, "Smiling Buddha" |
| Pokhran-II | May 1998 |
| ISRO | Formed in 1969 |
| Aryabhata (1975) | First Indian satellite |
| Chandrayaan-1 (2008) | Found water molecules on the Moon |
| Mangalyaan (2013) | Reached Mars orbit at the first attempt |
| Chandrayaan-3 (23 Aug 2023) | First soft landing near the Moon's south pole |
| Aditya-L1 (2023) | India's first solar mission |
Vikram Sarabhai is the father of India's space programme. 23 August is now National Space Day. Rakesh Sharma (1984) was the first Indian in space. Shubhanshu Shukla (2025, Axiom-4) was the first Indian to visit the International Space Station.
Inventions and who made them
| Invention or discovery | Person |
|---|---|
| Laws of motion, gravitation | Isaac Newton |
| Astronomical telescope | Galileo Galilei |
| Barometer | Evangelista Torricelli |
| Improved steam engine | James Watt |
| Electromagnetic induction, dynamo | Michael Faraday |
| Telephone | Alexander Graham Bell |
| Electric bulb, phonograph | Thomas Edison |
| Radio (wireless) | Guglielmo Marconi |
| Television | John Logie Baird |
| Telegraph | Samuel Morse |
| Aeroplane (1903) | Wright brothers |
| X-rays | Wilhelm Röntgen (first Physics Nobel, 1901) |
| Laser (1960) | Theodore Maiman |
| Dynamite | Alfred Nobel |
| Crescograph | J.C. Bose |
| Raman effect | C.V. Raman (Nobel 1930) |
| Chandrasekhar limit | S. Chandrasekhar (Nobel 1983) |
| Bose–Einstein statistics | Satyendra Nath Bose |
Einstein gave the theory of relativity. His Nobel Prize (1921) was for the photoelectric effect.
A laser gives an intense beam of one wavelength, in step (coherent). It is used in surgery, barcode scanners, CD/DVD players and optical-fibre communication. At CERN's Large Hadron Collider (2012), scientists found the Higgs boson, the "God particle". It explains why other particles have mass.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Particle or ray and its discoverer
The question says 'Who discovered the neutron / electron / proton / X-rays / radioactivity / nucleus?' or gives a match-the-column.
Particles: electron by J.J. Thomson, proton by Goldstein, neutron by Chadwick, nucleus by Rutherford.
Rays: X-rays by Röntgen, radioactivity by Becquerel, radium by the Curies.
Models: gold-foil experiment by Rutherford, orbits by Bohr.
Each name is fixed by history, so only recall and elimination work.
The canal rays that led to the discovery of the proton were observed by:
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Thomson found the electron in cathode rays.
Chadwick found the neutron in 1932.
Canal rays belong to Goldstein.
E. Goldstein
Device and its inventor
The question says 'Who invented the telephone / television / radio / electric bulb / dynamo / steam engine / aeroplane?'
Bell made the telephone. Baird made television. Marconi made radio. Edison made the bulb and phonograph. Morse made the telegraph.
Faraday found induction and the dynamo. Watt improved the steam engine. The Wright brothers built the aeroplane (1903). Galileo used the astronomical telescope. Torricelli made the barometer.
Drop near-names. J.C. Bose did pioneering microwave work, but the exam answer for radio is Marconi.
Inventions are fixed facts, so match each device to one name.
The television was invented by:
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Telephone is Bell.
Radio is Marconi.
Television is Baird.
John Logie Baird
Radioactivity and alpha, beta, gamma
The question asks 'Which radiation is most penetrating?', 'an alpha particle is ...', who found radioactivity, or which instrument detects radiation.
Radioactivity: Becquerel (1896). Radium and polonium: the Curies.
Alpha is a helium nucleus of 2 protons and 2 neutrons. Paper stops it. Beta is a fast electron. Gamma is an EM wave and the most penetrating.
The Geiger–Müller counter detects radiation. Libby's C-14 dating ages old remains.
The three rays differ in what they are and how far they go.
An alpha particle is identical to:
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An alpha particle has 2 protons and 2 neutrons.
That is the makeup of helium-4.
It has no electrons.
A helium nucleus
Fission or fusion; reactor parts
The question says 'The Sun's energy comes from ...', 'an atom bomb works on ...', 'control rods are made of ...', or compares the two bombs.
If a heavy nucleus splits, it is fission (reactors, atom bomb).
If light nuclei join, it is fusion (Sun and stars, hydrogen bomb).
Moderator slows neutrons (heavy water, graphite). Control rods absorb them (cadmium, boron). A coolant removes heat.
Both follow E = mc². The size of the nuclei tells you which process it is.
The energy of the Sun and stars is due to:
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Stars are mostly hydrogen.
Hydrogen nuclei join into helium.
This releases energy through E = mc².
Nuclear fusion
India's atomic and space milestones
The question asks for the first reactor, power station, satellite, Mars or Moon mission, the fathers of the two programmes, Pokhran tests or National Space Day.
Atomic: Bhabha is the father. Apsara 1956 was the first research reactor. Tarapur 1969 was the first power station. Pokhran tests were 1974 and 1998.
Space: Sarabhai is the father. ISRO 1969. Aryabhata 1975. Chandrayaan-1 2008. Mangalyaan 2013. Chandrayaan-3 landed on 23 Aug 2023. Aditya-L1 is the solar mission.
Rakesh Sharma was the first Indian in space (1984). Shubhanshu Shukla was the first Indian on the ISS (2025).
Each milestone is a fixed name, place or date.
India's first nuclear power station was set up at:
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Apsara was a research reactor, at Trombay (1956).
A power station came later, in 1969.
Tarapur, Maharashtra (1969)
Modern physics miscellany
One-line fact questions on E = mc², the 'God particle', laser properties, carbon dating or first Nobel Prizes.
E = mc² is Einstein's. Mass can turn into energy. It is the base of fission and fusion.
The Higgs boson (the 'God particle') was found at CERN's LHC in 2012.
Laser light has one wavelength, is coherent and is highly directional.
Carbon-14 dating is Libby's method for fossils and old organic objects.
Each fact is a short, fixed statement.
The 'God particle' whose existence was confirmed at CERN in 2012 is the:
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It explains why particles have mass.
It was predicted by Peter Higgs.
Higgs boson
Formula sheet
Einstein
Shortcuts that save time
Fission means split: heavy nucleus to lighter ones (reactors, atom bomb). Fusion means join: light nuclei to a heavier one (Sun, hydrogen bomb).
The energy of the Sun comes from:
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The Sun is made mostly of hydrogen.
Hydrogen nuclei join into helium.
Joining is fusion.
Nuclear fusion
Electron: Thomson (1897). Nucleus: Rutherford. Neutron: Chadwick (1932). Proton rays: Goldstein.
Mistakes to avoid
Where most students lose marks on this subtopic.
Saying Einstein won the Nobel Prize for relativity.
It was for the photoelectric effect (1921).
Saying the moderator absorbs neutrons.
The moderator slows them. Control rods (cadmium, boron) absorb them.
Crediting Edison with the telephone.
The telephone was Alexander Graham Bell's. Edison made the bulb and phonograph.
Saying a reactor uses fusion.
Reactors use fission. Fusion powers the Sun and the hydrogen bomb.
Taking gamma rays as particles.
Gamma rays are electromagnetic waves, the most penetrating of the three.
Quick revision
Read this the night before the exam.
Electron: Thomson. Proton: Goldstein (canal rays). Neutron: Chadwick. Nucleus: Rutherford. Orbits: Bohr.
Radioactivity: Becquerel. Radium and polonium: the Curies. C-14 dating: Libby. Counter: Geiger.
Alpha is a helium nucleus. Beta is an electron. Gamma is an EM wave and the most penetrating.
Fission is heavy splitting (reactor, atom bomb). Fusion is light joining (Sun, H-bomb). E = mc².
Bhabha is atomic, Sarabhai is space. Apsara 1956, Tarapur 1969, Pokhran 1974 and 1998.
Aryabhata 1975. Chandrayaan-3 landed on 23 Aug 2023. Mangalyaan reached Mars orbit at the first attempt.
Practice: 22 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.