Refraction
Waves change direction when their speed changes, bending light through air and glass, and sound and swell through the sea.
Refraction is the bending of a wave when it passes into a region where it travels at a different speed. One part of the wavefront slows down or speeds up before the rest, so the whole front swings round. A straw in a glass of water looks broken at the surface for exactly this reason.
The atmosphere does it too. Air thins with height, and light travels very slightly faster in thinner air, so sunlight and starlight curve gently downwards on their way to us. At the horizon the bending is usually about 34 arcminutes, slightly more than the Sun's apparent width. So when the Sun's lower edge seems to touch the horizon at sunset, the true Sun is already just below it, and every day of sunshine lasts a little longer than it would without air. In layers of very cold air under warmer air the bending can be far stronger, producing mirages and the Novaya Zemlya effect, in which the Sun appears while it is still well below the horizon.
Sound and water waves refract as well. Ocean swell slows in shallow water and bends to break almost parallel to a beach, and swells bend around islands. In the deep ocean, sound bends towards the depth where it travels slowest, where it can be trapped in a channel and carried for thousands of kilometres.
Coming up in future stories
- The Fish That Shoots Insects and Corrects for Refraction 1764 → today · mangroves of Southeast Asia and the lab
- The Ocean's Hidden Telephone Line 1944 · Atlantic Ocean off Woods Hole
- The Sailors Who Mapped Waves With Sticks 19th century · Marshall Islands
- The Sun Rose Two Weeks Early 1597 · Novaya Zemlya, Arctic Russia
- Two Scholars Solved the Rainbow at the Same Time c. 1300–1311 · Tabriz, Ilkhanate Iran, and Freiberg, Germany