Rainbow in a Water Droplet

Theory

Sunlight enters a spherical droplet, refracts, reflects once on the back surface and refracts again on the way out. Because water’s index varies with colour, each colour exits at a slightly different angle — about 42° for red and 40° for violet relative to the incoming light: the primary rainbow (Descartes, Newton).

What to observe

• The back of the drop is silvered here so you can follow the single internal reflection.
• The light comes back toward the Sun side, spread into colours at about 40°–42°.

Try this

• Move the light source up and down to change the impact parameter.
• Increase the dispersion of the drop to exaggerate the effect.

Also available in: Arco-íris numa gota de água (Português) · Arcoíris en una gota de agua (Español)