Chapter 28: Quantum Physics

Photons are discrete energy packets whose energy relates directly to frequency through Einstein's relation, with the electronvolt serving as a practical unit for measuring these minuscule energy quantities. The photoelectric effect demonstrates light's particle behavior when it strikes metal surfaces and releases electrons; this phenomenon revealed that increasing light intensity merely changes the emission rate rather than electron speed, while only frequency changes affect the maximum kinetic energy of ejected photoelectrons. Einstein's photoelectric equation balances incoming photon energy against the work function and resulting kinetic energy. Beyond energy, photons carry momentum despite lacking mass, enabling light to exert measurable pressure on physical objects. The chapter then explores atomic structure through the lens of quantized energy levels, where electrons occupy only specific, discrete states within atoms. When electrons transition between these levels, they emit or absorb photons with energies precisely matching the energy gap, producing the characteristic line spectra observed in emission and absorption experiments. The chapter concludes by establishing that particles exhibit wave properties analogous to light's particle properties. De Broglie's hypothesis proposes that all moving matter possesses an associated wavelength inversely proportional to momentum, a prediction confirmed through electron diffraction experiments showing wave-like interference patterns. However, macroscopic objects possess wavelengths too small to observe diffraction effects. Together, these concepts establish wave-particle duality as a fundamental principle: electromagnetic radiation and matter both exhibit wave characteristics when propagating through space yet behave as discrete particles when interacting with other systems, unifying the behavior of light and matter under quantum mechanical principles.