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Modern physics emerged in the early 20th century, fundamentally transforming our understanding of reality. This hub covers special relativity, the photoelectric effect, wave-particle duality, quantum mechanics, de Broglie matter waves, nuclear physics, and radioactive decay.
Special relativity rests on two postulates: (1) The laws of physics are identical in all inertial reference frames. (2) The speed of light in a vacuum (c = 3×10&sup8; m/s) is constant for all observers, regardless of the motion of the source or observer.
Consequences: time dilation (moving clocks run slow), length contraction (moving objects appear shorter in the direction of motion), and mass-energy equivalence: E = mc².
When light hits a metal surface, electrons are emitted only if the light frequency exceeds a threshold value (f&sub0;). Key observations: (1) Increasing intensity increases the number of electrons, not their energy. (2) Increasing frequency increases electron kinetic energy. Einstein (1905) explained this using photons: E = hf, where h = 6.63×10&sup-34; J·s (Planck constant). This showed light has particle-like (quantized) properties.
Louis de Broglie (1924) proposed that all particles also have wave properties: λ = h/p = h/(mv). Electrons diffract through crystals (Davisson-Germer experiment), confirming matter waves. The double-slit experiment shows that even individual electrons create an interference pattern.
Atomic nuclei contain protons and neutrons (nucleons). Nuclear binding energy holds them together, releasing energy when nuclei are split (fission) or joined (fusion). Radioactive decay types: alpha (α) — helium nucleus emitted; beta (β) — electron or positron emitted; gamma (γ) — high-energy photon emitted. All governed by E = mc².
Key formulas with definitions, variables, and SI units.
Unit: J (Joules)
Unit: J (Joules)
Unit: m (metres)
Unit: s (seconds)
Unit: J (Joules)
Unit: count
Common questions about this topic, answered clearly.
Einstein special relativity (1905) states that the speed of light is the same for all observers and that the laws of physics are identical in all inertial frames. Key consequences are time dilation, length contraction, and E = mc².
When light above a threshold frequency hits a metal, it ejects electrons. The energy of each electron depends on frequency (not intensity) of light. Einstein explained this by treating light as discrete photons with energy E = hf, winning the 1921 Nobel Prize.
All matter and radiation exhibit both wave and particle properties. Electrons interfere like waves (wave nature) but transfer energy in discrete packets (particle nature). The de Broglie wavelength λ = h/mv applies to all particles.
Fission splits a heavy nucleus (e.g. U-235) into smaller fragments, releasing energy. Fusion joins light nuclei (e.g. hydrogen isotopes) into a heavier nucleus, releasing even more energy per kilogram. Both obey E = mc².