Spectra
Motivation
(Source: www.harmsy.freeuk.com/fraunhofer.html)
The spectral lines were discovered thanks to decomposition of solar or star light on spectrum. There are dark lines in places of missing colours, or there are individual colours in analyzed light. Questions such as "Why do we observe such lines in light spectrum? Can we use them for chemical analysis?" had been for a long time unasnwered and mysteroius until the quantum physics was born.