Splitting White Light: Prisms and Rainbows

Lesson 7 of 10 · 7 min · Free, no account needed

What you will learn

White light is a mix of all colours. Glass bends each colour a little differently: dispersion. Violet bends most, red least.
The visible spectrum runs from red (about 700 nm) to violet (about 400 nm).
A rainbow is a circle about 42° around the point opposite the Sun, so you see one with the Sun behind you and lower than 42°.

Why is a rainbow always opposite the Sun, and never seen at noon in summer?
First: the Sun is low behind you and rain falls ahead. Draw the rainbow.
Shine white light into a prism.
Then try one colour on its own.
Tune a laser's colour to hit each target.
The dial sets the wavelength in nanometres (nm).
Dispersion and the spectrum.
Dispersion: the refractive index depends on colour, so each colour bends by a different amount. Violet bends most, red least.
The visible spectrum: red, orange, yellow, green, blue, violet, from about 700 nm down to about 400 nm wavelength.
A rainbow: sunlight bends into raindrops, reflects off the back and bends out again. Each colour leaves at its own angle, about 42° from the point opposite the Sun.
Simplified: our prism glass spreads colours about five times more than real glass, so you can see the fan.
Raise the Sun. What happens to the rainbow?
The dotted line is your drawing.
Game: Rainbow hunter. Four places, four times of day.
AI: a prism for sound.
A prism spreads light by frequency. A spectrogram does the same to sound.
Which colour does the prism bend the most?
In glass the refractive index is a little bigger for short wavelengths: about 1.53 for violet against 1.51 for red in ordinary glass. A bigger n means a bigger bend at each surface, so violet ends up turned the most.
Put your rainbows in your album.

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