Modern Physics: Atoms, Radioactivity, Nuclear Energy, Semiconductors & Lasers

Modern Physics questions appear 2-3 times in every Kerala PSC paper. Focus areas include radioactivity, nuclear energy, and semiconductor applications. This note covers exam-critical facts in table format.

1. Atomic Structure — Key Concepts

Scientist	Model/Discovery	Year
J.J. Thomson	Discovered electron; Plum Pudding Model	1897
Ernest Rutherford	Nuclear model (gold foil experiment); discovered proton	1911
Niels Bohr	Planetary model with quantized orbits	1913
James Chadwick	Discovered neutron	1932
Erwin Schrodinger	Quantum mechanical model (wave equation)	1926

Subatomic Particles

Particle	Charge	Mass	Location
Proton	+1	1.67 x 10^-27 kg (1 amu)	Nucleus
Neutron	0 (neutral)	~1 amu (slightly more than proton)	Nucleus
Electron	-1	9.1 x 10^-31 kg (1/1836 of proton)	Orbits around nucleus

Key Terms

Term	Definition
Atomic number (Z)	Number of protons
Mass number (A)	Protons + Neutrons
Isotopes	Same Z, different A (same element, different mass)
Isobars	Same A, different Z (different elements, same mass)
Isotones	Same number of neutrons

2. Radioactivity

Feature	Detail
Discovered by	Henri Becquerel (1896)
Term coined by	Marie Curie
Definition	Spontaneous emission of radiation from unstable atomic nuclei
Types of radiation	Alpha, Beta, Gamma

Three Types of Radiation

Property	Alpha (a)	Beta (b)	Gamma (g)
Nature	Helium nucleus (2p + 2n)	Electron (or positron)	Electromagnetic wave
Charge	+2	-1 (or +1 for positron)	0
Mass	4 amu	~0 (1/1836 amu)	0
Speed	~5% of light	~90% of light	Speed of light
Penetration	Least (stopped by paper)	Medium (stopped by aluminium sheet)	Highest (stopped by thick lead/concrete)
Ionization power	Highest	Medium	Least

Radioactive Decay Laws

Concept	Detail
Half-life	Time for half the radioactive atoms to decay
Activity	Rate of disintegration (measured in Becquerel or Curie)
1 Becquerel (Bq)	1 disintegration per second
1 Curie (Ci)	3.7 x 10^10 disintegrations per second

Important Radioactive Elements

Element	Half-life	Use
Carbon-14	5,730 years	Carbon dating (archaeology)
Uranium-238	4.5 billion years	Nuclear fuel; geological dating
Cobalt-60	5.27 years	Cancer treatment (radiation therapy)
Iodine-131	8 days	Thyroid treatment/diagnosis
Radium-226	1,600 years	Historically used in cancer therapy

3. Nuclear Fission

Feature	Detail
Definition	Heavy nucleus splits into lighter nuclei + energy
Discovered by	Otto Hahn and Fritz Strassmann (1938)
Common fuel	Uranium-235, Plutonium-239
Trigger	Neutron bombardment
Chain reaction	Each fission releases 2-3 neutrons that cause more fissions
Energy per fission	~200 MeV
Application	Nuclear power plants, atomic bomb

Nuclear Reactor Components

Component	Function
Fuel	U-235 or Pu-239 (fuel rods)
Moderator	Slows down neutrons (heavy water, graphite, light water)
Control rods	Absorb excess neutrons to control reaction (Cadmium or Boron)
Coolant	Removes heat (water, heavy water, liquid sodium)
Shielding	Thick concrete/lead to prevent radiation leakage

India’s Nuclear Power Plants

Plant	State	Type
Tarapur	Maharashtra	BWR (Boiling Water Reactor) — first in India (1969)
Rawatbhata (RAPS)	Rajasthan	PHWR
Kalpakkam (MAPS)	Tamil Nadu	PHWR
Narora	Uttar Pradesh	PHWR
Kaiga	Karnataka	PHWR
Kudankulam	Tamil Nadu	VVER (Russian-designed, largest)
Kakrapar	Gujarat	PHWR

4. Nuclear Fusion

Feature	Detail
Definition	Light nuclei combine to form heavier nucleus + enormous energy
Example	Hydrogen nuclei fuse to form Helium (in Sun/stars)
Temperature required	Millions of degrees Celsius (thermonuclear)
Energy source of Sun	Hydrogen fusion (4H to He)
Application	Hydrogen bomb (uncontrolled); future: fusion reactors (ITER project)
Advantage over fission	Virtually unlimited fuel (hydrogen from water); less radioactive waste
Challenge	Containing plasma at extreme temperatures

Fission vs Fusion

Feature	Fission	Fusion
Process	Splitting heavy nuclei	Combining light nuclei
Fuel	Uranium, Plutonium	Hydrogen isotopes (Deuterium, Tritium)
Energy per reaction	~200 MeV	~17.6 MeV (but per unit mass, much more)
Temperature	Can occur at room temp (with neutron)	Millions of degrees needed
Waste	Highly radioactive	Minimal radioactive waste
Controlled use	Nuclear power plants	Not yet achieved commercially

5. X-Rays

Feature	Detail
Discovered by	Wilhelm Roentgen (1895)
Nature	Electromagnetic radiation (wavelength 0.01-10 nm)
Production	When high-speed electrons strike a metal target (tungsten)
Properties	Penetrate soft tissue; blocked by dense material (bone, lead)
Uses	Medical imaging, baggage scanning, crystallography
Harmful effects	Can cause cell damage, mutations, cancer with overexposure

Types of X-rays

Type	Use
Soft X-rays	Lower energy; used in mammography
Hard X-rays	Higher energy; penetrate more; used in CT scans

6. Laser

Feature	Detail
Full form	Light Amplification by Stimulated Emission of Radiation
Principle	Stimulated emission (Einstein, 1917 theory)
First laser built by	Theodore Maiman (1960) — Ruby laser
Properties	Monochromatic, coherent, highly directional, intense

Types of Lasers

Type	Medium	Application
Ruby laser	Solid (chromium-doped aluminium oxide)	Holography
He-Ne laser	Gas (helium-neon)	Bar code scanners, alignment
CO2 laser	Gas	Cutting, welding, surgery
Semiconductor laser (diode laser)	Solid (GaAs)	CD/DVD players, fiber optics, laser pointers
Nd:YAG laser	Solid	Eye surgery (LASIK), industrial cutting

Applications of Laser

• Eye surgery (LASIK, retinal repair)
• Fiber optic communication
• CD/DVD/Blu-ray reading
• Industrial cutting and welding
• Military (range-finding, guided weapons)
• Holography (3D imaging)
• Barcode scanning
• Laser printing

7. Semiconductors

Feature	Detail
Definition	Materials with conductivity between conductors and insulators
Examples	Silicon (Si), Germanium (Ge), Gallium Arsenide (GaAs)
Band gap	Small (~1 eV for Si) — electrons can jump to conduction band
Conductivity increases with	Temperature (opposite to metals)

Types of Semiconductors

Type	Doping	Majority Carriers	Dopant Examples
Intrinsic (pure)	None	Equal electrons and holes	—
N-type	Pentavalent impurity	Electrons	Phosphorus (P), Arsenic (As)
P-type	Trivalent impurity	Holes	Boron (B), Aluminium (Al)

P-N Junction (Diode)

Feature	Detail
Forward bias	P connected to +ve, N to -ve; current flows
Reverse bias	P connected to -ve, N to +ve; almost no current
Application	Rectifier (AC to DC conversion)
Depletion region	Thin layer at junction with no free carriers

Transistor

Feature	Detail
Definition	Three-layer semiconductor device (NPN or PNP)
Invented by	Shockley, Bardeen, Brattain (1947, Bell Labs)
Functions	Amplification, switching
Used in	All electronic devices, computers, phones

8. LED (Light Emitting Diode)

Feature	Detail
Principle	Electroluminescence — electrons recombine with holes, emit photons
Material	GaAs (infrared), GaAsP (red/yellow), GaN (blue/white)
Advantages	Low power, long life, no heat, small size
Applications	Displays, lighting, indicators, traffic signals
Blue LED	Invented by Akasaki, Amano, Nakamura (Nobel Prize 2014)

9. Solar Cell (Photovoltaic Cell)

Feature	Detail
Principle	Photovoltaic effect — light energy converted to electrical energy
Material	Silicon (most common), GaAs, CdTe
Structure	P-N junction; sunlight creates electron-hole pairs
Efficiency	15-22% for commercial silicon cells
Applications	Rooftop panels, satellites, calculators, street lights
Advantages	Renewable, no pollution, low maintenance
Limitation	Intermittent (no power at night), high initial cost

India’s Solar Energy

Feature	Detail
National Solar Mission	Launched 2010 (part of National Action Plan on Climate Change)
Target	300 GW solar by 2030
Largest solar park	Bhadla Solar Park, Rajasthan (2,245 MW)
International Solar Alliance	HQ in Gurugram; co-founded by India and France (2015)

10. PSC Exam Quick-Fire Facts

• Radioactivity discovered by: Henri Becquerel (1896)
• Radium discovered by: Marie and Pierre Curie
• Marie Curie: Only person to win Nobel in two different sciences (Physics 1903, Chemistry 1911)
• X-rays discovered by: Roentgen (also called Roentgen rays)
• First nuclear reactor: Chicago Pile-1 (Enrico Fermi, 1942)
• India’s first nuclear test: Pokhran-I (Smiling Buddha), 1974
• India’s second nuclear tests: Pokhran-II (Operation Shakti), 1998
• Father of Indian Nuclear Programme: Homi J. Bhabha
• Father of Indian Missile Programme: Dr. A.P.J. Abdul Kalam
• LED does NOT use filament (unlike incandescent bulb)
• Solar cell converts sunlight DIRECTLY to electricity (no turbine)
• Unit of radioactivity: Becquerel (SI) or Curie (old)
• Unit of radiation dose: Gray (absorbed) or Sievert (biological effect)