Evidence for the Quantized Electronic Energy Levels: Atomic Spectra

2.3.3 Evidence for the Quantized Electronic Energy Levels: Atomic Spectra

The speed of light is not the same in every medium — it depends on the medium's optical properties. When light passes from one medium into another (say, from air into a glass prism), it bends, or refracts, away from its original path. Different wavelengths bend by different amounts: shorter wavelengths (like violet) bend more than longer wavelengths (like red). This is why passing ordinary white light through a prism spreads it out into a continuous band of colours — the familiar rainbow, or continuous spectrum, running smoothly from violet through to red with no gaps. A rainbow in the sky is produced the same way, by water droplets acting as countless tiny prisms.

Ordinary sunlight or the light from an incandescent bulb produces exactly this kind of continuous, unbroken spectrum. Atoms behave very differently. When an atom absorbs energy — by being heated strongly or subjected to an electric discharge — it is pushed into a higher, less stable energy state. To return to a lower, more stable state, it must give that energy back, and it does so by emitting electromagnetic radiation. …