The Caustic in the Rain: teacher key

Rainbows, grade 11 edition, written for 11th grade. A rainbow is a bright line where light rays pile up. Trace how scholars, Descartes, Newton and Young used it to test what light is, and why drops, waves and angles matter.

Read the book. Print this page for the key.

Questions and answers

1. What do physicists call a bright line where many light rays pile up at the same angle?
   • A caustic (answer)
   • A spectrum
   • A glory
2. Which person explained supernumerary bows as interference of light waves?
   • Descartes
   • Thomas Young (answer)
   • Aristotle
3. Why can the simple ray picture not explain supernumerary bows?
   • Rays cannot show how light waves add and cancel (answer)
   • Rays only work at night
   • Raindrops are square
4. A rainbow forms where the exit angle of the rays reaches a turning point. What does this explain?
   • Why the arc is bright (answer)
   • Why the sky is blue
   • Why moonbows are red
5. In this passage, 'dispersion' means what?
   • Light spread out by wavelength (answer)
   • Rain falling in drops
   • Light reflecting off a cloud

Words

• caustic: bright ray pile-up
• dispersion: spreading by wavelength
• interference: waves adding together
• supernumerary: extra, additional
• antisolar: opposite the sun