Physical Phenomena That Launched Quantum Physics
Physical Phenomena That Motivated the Birth of Quantum Physics
Explain TWO different physical phenomena that motivated the birth of quantum physics at the beginning of the 20th century.
1. Atomic Spectra
Classical physics could not explain the discrete lines observed in atomic emission and absorption spectra.
Quantum interpretation by Bohr: Electrons orbit the nucleus in quantized energy levels, and transitions between these levels result in the emission or absorption of photons with specific energies, leading to discrete spectral lines.
2. Black-Body Radiation
Classical physics predicted that a black body would emit infinite energy at short wavelengths (the ultraviolet catastrophe).
Quantum interpretation: Max Planck proposed that energy is quantized and can only be emitted or absorbed in discrete amounts (photons), leading to the correct black-body radiation spectrum.
3. The Photoelectric Effect
Hertz's experiments showed that light shining on a metal surface ejects electrons. Classical wave theory could not explain why light below a certain frequency (the threshold frequency) did not cause electron emission, regardless of intensity.
Quantum interpretation by Einstein: Light is quantized into photons, each with energy proportional to its frequency. Only photons with energy greater than the work function can eject electrons.
4. Diffraction of Massive Particles
Davisson-Germer experiment (1927): Electrons were observed to diffract when passing through a crystal, similar to light waves. Classical particle theory could not explain this wave-like behavior.
Quantum interpretation by de Broglie: Particles have wave-like properties, with a wavelength given by λ = h/p.
Interpretations of Quantum Mechanics
COPENHAGEN: Quantum systems exist in superpositions of multiple possibilities; when measured, the wave function collapses to one result. An alternative is the many-worlds interpretation: every possible outcome occurs in parallel universes.
Blackbody Radiation
Blackbody radiation is emitted by an ideal object that absorbs all incident radiation at any wavelength and re-emits it in thermal equilibrium, producing a continuous spectrum that depends only on its temperature. Its peak shifts to shorter wavelengths as temperature increases, according to Wien's law. Classical physics predicted the ultraviolet catastrophe, i.e., a divergence of energy at short wavelengths. Max Planck resolved this by introducing energy quantization: E = hν, marking the beginning of quantum physics.