Physics of Scuba Diving
Light
Diving with a flashlightWhile underwater, scuba divers cannot rely on their eyes to be as accurate as they might be above water.

When a scuba diver looks up into the air above the body of water he is diving in, any objects he sees will be in the wrong place. This is because your eyes automatically assume that the light reaching them as traveled along a straight path. Underwater, this is not the case.

This is all due to the fact that light travels slower in water than it does in air. Since this is true, any light that enters the water from the air is bent.

Refraction

Also, divers underwater can only see a limited amount of the surface above them. The critical angle, which is the angle that divides where light is either internally reflected or passed through bent, of light passing from water into air is 49 degrees. The diver underwater will be unable to see anything that is less than 49 degrees above the horizon. This is clearly shown in the picture above.

Diving with stingraysHere are some quick facts about light underwater:
  • Colors begin to drop out at varying depths as you dive down.

  • Red disappears first at about 30 feet down because it has a longer wavelength.

  • Only blues and greens are left at depths below 100 feet. This is why many divers use flashlights, which allow them to see the other colors.

  • When divers use masks, it causes objects underwater to be 30% larger than they really are.

  • All images courtesy Corbis Stock Photography. Created in 2003 by David Baxter & Timothy Schmidt.