Light and colour, in 3D

What the simulation shows

• White light entering a glass prism and fanning out into the colours of the spectrum (red bends least, violet most).
• A raindrop: sunlight bends in, reflects off the back and comes out split into colours at about 42°.
• Parallel light through a convex lens, bending to meet at a focus.
• The lens inside an eye: the image of an arrow lands upside down on the retina.
• A beam bouncing off a mirror: angle in equals angle out.
• Light zigzagging along a glass fibre, trapped by total internal reflection.
• Sunlight through the air: blue light scatters in every direction (filling the sky), red goes straight on.
• The Sun low: its light crosses much more air, the blue is scattered away and only red and orange arrive.
• A callout: 300,000 kilometres a second, seven and a half times round the Earth each second.

The real numbers it runs on

The simulation is built on these facts, so the narration and the picture always agree:

• White light is a mix of all colours. Glass and water bend (refract) light, violet slightly more than red, so a prism or raindrop spreads them into a spectrum.
• In a raindrop sunlight bends in, reflects off the back and bends out; the colours come back at about 42° from the point opposite the Sun, which is why a rainbow is a circle around the shadow of your head.
• A convex lens bends light to a focus. The eye's lens focuses an upside-down image on the retina, and the brain turns it the right way up.
• A mirror reflects light at the same angle it arrives.
• Light inside a glass fibre hits the wall at a shallow angle and is totally reflected, so it stays trapped even round bends: that's how the internet crosses oceans.
• Air scatters blue light about five times more than red (Rayleigh scattering), so blue reaches us from every direction: the sky looks blue. At sunset light crosses many times more air; most blue is scattered away and the Sun looks red.
• Light travels at about 300,000 kilometres a second: about 7.5 times round the Earth in one second; sunlight takes 8 minutes 20 seconds to reach us.

The script this video was made from

“Why the sky is blue”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

Make your own

1. Type a question, like “Why the sky is blue”, or your own angle on it.
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