Physics

Inside the atom

An atom has a tiny dense nucleus (protons + neutrons) with electrons moving around it. The particles were found one by one:

• 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 passed straight through, a few bounced back, so nearly all mass and positive charge sit in a tiny centre. Bohr (1913) added fixed orbits for electrons.

Radioactivity

Henri Becquerel (1896) discovered that uranium salts give out invisible penetrating rays — radioactivity, a property of heavy unstable nuclei. Marie and Pierre Curie discovered polonium and radium; Marie Curie won Nobel Prizes in both Physics (1903) and Chemistry (1911) — the only person with Nobels in two different sciences. The alpha ray is a helium nucleus (least penetrating, stopped by paper), beta is a fast electron (stopped by aluminium), and gamma is an electromagnetic wave (most penetrating, needs thick lead). A Geiger–Müller counter detects radiation. Willard Libby's carbon-14 dating finds the age of old organic remains from the C-14 left in them.

Fission, fusion and reactors

• Nuclear fission: a heavy nucleus such as U-235 splits into two smaller nuclei when hit by a neutron, releasing energy and more neutrons — a chain reaction. Uncontrolled → atom bomb (Hiroshima, 1945). Controlled in reactors → electricity.
• Nuclear fusion: two light nuclei (hydrogen isotopes) join into a heavier one — this powers the Sun and stars, and the hydrogen bomb (far more destructive than a fission bomb).
• Both follow Einstein's E = mc²: a little lost mass becomes a lot of energy.
• In a reactor: the moderator (heavy water, graphite) slows neutrons, control rods (cadmium, boron) absorb neutrons to control the chain reaction, and a coolant carries the heat away. Reactors use fission, not fusion.

India's atomic and space programmes

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. Apsara (1956) was India's (and Asia's) first research reactor; Tarapur in Maharashtra (1969) is India's first nuclear power station. India's first nuclear test was at Pokhran in 1974 ("Smiling Buddha"); Pokhran-II followed in May 1998. In space, Vikram Sarabhai is the father of the Indian space programme and ISRO was formed in 1969. Aryabhata (1975) was the first Indian satellite. Chandrayaan-1 (2008) found water molecules on the Moon; Mangalyaan (Mars Orbiter Mission, 2013) made India the first country to reach Mars orbit in its first attempt; Chandrayaan-3 (23 August 2023) made India the first nation to soft-land near the Moon's south pole — the date is now National Space Day. Aditya-L1 (2023) is India's first solar mission. Rakesh Sharma (1984) was the first Indian in space; Shubhanshu Shukla (2025, Axiom-4) the first Indian to visit the International Space Station.

Inventions — who did what

Quick revision

• Electron–Thomson, proton–Goldstein (canal rays), neutron–Chadwick, nucleus–Rutherford, orbits–Bohr.
• Radioactivity–Becquerel; radium/polonium–Curies; C-14 dating–Libby; counter–Geiger.
• α = helium nucleus, β = electron, γ = EM wave (most penetrating).
• Fission = heavy splits (reactor, atom bomb); fusion = light join (Sun, H-bomb); E = mc².
• Bhabha (atomic), Sarabhai (space); Apsara 1956, Tarapur 1969, Pokhran 1974/1998.
• Aryabhata 1975, Chandrayaan-3 land 23-08-2023, Mars orbit first try 2014.