Changing Theories of Light

Newton’s corpuscular picture treated light as particles and could explain straight-line propagation and reflection. His authority and the difficulty of observing ordinary-light diffraction contributed to its acceptance. Huygens described propagation through advancing wavefronts; a wave approach could also account for refraction and diffraction.

In the historical corpuscular account, refraction into a more optically dense medium required light to speed up, whereas the wave account predicted a lower speed. Evidence about propagation speed could therefore distinguish them. Avoid judging either theory by the reputation of its proposer: ask what observations it predicts.

Young’s two slits produced alternating bright and dark fringes. In the wave account, the two coherent contributions reinforce or cancel depending on path difference. Persistent cancellation is difficult to explain with a simple stream of independent corpuscles. Acceptance of the wave account was delayed until experimental evidence accumulated.

Fizeau sent light through a gap in a rotating toothed wheel to a distant mirror and back. At a suitable wheel speed the return met the adjacent tooth and was blocked. The measured travel time was finite, allowing a terrestrial speed estimate. For N teeth and N gaps, the first gap-to-tooth blocking interval is $1/(2Nf)$, matched to the round-trip time.

Maxwell’s equations connected changing electric and magnetic fields and predicted waves with speed $1/\sqrt{\mu_0\epsilon_0}$. Its agreement with measured light speed supported the identification of light as electromagnetic radiation. The fields are transverse and mutually perpendicular in a plane wave.

Permittivity relates to electric fields produced by charge; permeability relates to magnetic fields produced by current. Hertz generated and detected radio waves, demonstrated wave behaviours and measured a speed consistent with light. The result supported a common electromagnetic nature across visible light and radio frequencies.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.