Sunlight in the sea: zones, colours and life
Visible light is a small part of the electromagnetic spectrum. Its colours, in order, are red, orange, yellow, green, blue, indigo and violet, and they differ in wavelength (about 400 to 700 nm, extra). When sunlight meets water, part is reflected, part is absorbed by water, suspended matter and plants, and part is scattered. More is reflected when the rays come at a slant; most enters when they come straight down (90°). Light gets weaker with depth, which defines three zones: sunlit, twilight and dark. According to the book, infrared is absorbed within the first 10 cm, more than half of the visible light is absorbed in the top 10 m, and in clear tropical water only about 1% of visible light, mostly blue, reaches 100 m. Red and orange (long wavelengths) are absorbed first; blue light goes deepest. Algae, plants and phytoplankton live near the surface, deeper in clear water and nearer the surface in turbid water. Coral reefs grow in warm, shallow, sunny water because algae living in the coral need light. The dark zone is not empty of life. Near the poles the Sun is low in winter, so photosynthesis falls.
| Depth in clear tropical water | Visible light left (book, rounded) |
|---|---|
| surface | 100% |
| 10 m | less than half |
| 100 m | about 1%, mostly blue |
Torch in a tub, with a grown-up. In a dim room, shine a torch from the side of a clear tub of tap water and describe the beam. Then add a few drops of milk, stir, and describe the beam again. Hold red and blue cellophane (or coloured paper) over the torch and describe which colour looks brighter through more water. Never look at the Sun or shine the torch at faces.