Door Prisms ~ Sunlight floods into an office at Basel, Switzerland. The chamfered glass door edge acts as a prism to mimic Newton’s experiments in the late 1660s on the ‘celebrated phenomenon of colours’. White light disperses into its component colours.

Images by Eric Guicherit.
©Eric Guicherit, shown with permission

At right, plane waves enter glass (or water or ice) from air.

The six highlighted wave crests travel the distance AB in air. The same six crests in the water travel a shorter distance CD. The wave’s direction of travel is altered.

See an animation - try it for diamond's refractive index of 2.42.

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Why is light refracted? The key is that the speed of light is not constant, it slows significantly in dense media like water and glass. In ultra-dense diamond it crawls along at a mere 77,000 miles per second, only 41% of its vacuum velocity.

A light wave entering a dense medium interacts with its atoms. When the wave reaches an atom it is absorbed and induces small oscillations in the atom’s electron cloud. The atom then re-emits light at the exact same frequency which then proceeds as an outward wave at vacuum velocity. This radiation then interacts with another atom and so on… The small delays in re-radiation at each atom collectively appear as an overall reduction in the light’s velocity.

The speed reduction produces the refraction.