Prism

A triangular prism splits white light into the visible spectrum. Each color (wavelength) has a slightly different refractive index:

$$n_{\text{red}} < n_{\text{orange}} < \cdots < n_{\text{violet}}$$

When light refracts through two surfaces, these tiny differences compound: red bends least, violet bends most. That's chromatic dispersion, the same physics behind rainbows and why early telescopes had colored fringes.

Drag vertically to change the incident angle and watch the spectrum spread.