Introduction

Every optical phenomenon studied so far in this course -- reflection, refraction, interference, diffraction, and polarization -- was explained by treating light as an electromagnetic wave: a pair of mutually perpendicular, oscillating electric and magnetic fields, both perpendicular to the direction of travel. This wave picture works equally well for the entire electromagnetic spectrum, from X-rays and gamma rays through ultraviolet, visible, and infrared light, to microwaves.

But not every phenomenon involving radiation fits this picture. The spectrum of black-body radiation -- the characteristic glow of a hot object, spread across wavelengths -- could not be explained by classical wave theory at all. Such failures crop up specifically when radiation INTERACTS with matter, and resolving them needed an entirely new idea: quantum physics.

The breakthrough came from Max Planck, who proposed that the energy carried by electromagnetic radiation is not continuous but comes in discrete packets, or QUANTA. Planck modelled the atoms in a hot object as tiny oscillators, each emitting radiation only in fixed packets $E=nh\nu$, where $\nu$ is the oscillator's frequency, $h$ is a new fundamental constant (now called Planck's constant), and $n$ is a whole number. Crucially, an oscillator only radiates when it jumps from one allowed (quantized) energy level down to a lower one -- there is no continuous leakage of energy in between. Higher-frequency oscillators emit larger packets of energy.

Planck introduced this quantization purely to match the observed black-body spectrum, without offering a deeper physical reason for it. It was this same idea, extended by Einstein, that became the key to understanding a second and equally puzzling phenomenon: the photoelectric effect, taken up in the next section.