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Deep Blue

Science Snack
Deep Blue
Underwater, what you see is not what you get.
Deep Blue
Underwater, what you see is not what you get.

Submerge yourself in underwater color perception: Use photographs and colored light bulbs to see how changing light conditions affect the apparent color of deep-sea dwellers.

Photos: Rainbow Reef, Fiji, David Burdick

Tools and Materials
  • Red, green, and blue screw-in light bulbs, one of each color (you can use any type—CFL, LED, or incandescent—though older incandescent bulbs may not work as well)
  • Three screw-in light sockets
  • Power strip with at least three parallel outlets
  • Power source (and extension cord if needed)
  • Printed color images of underwater scenes showing multicolored fish and other organisms (make sure there are red and yellow creatures among them)
  • Darkened room
  1. Set up your light source: Screw the red, green, and blue bulbs into the light sockets.
  2. Plug the three bulbs into the power strip and turn them all on.
To Do and Notice

Spread out the colored images next to the strip of glowing light bulbs, and then turn off the room lights. Notice the colors in the images. Do the colors look any different under the combined red, green, and blue bulbs, compared with the white lights of the room?

Turn off only the red light bulb and notice the colors in the images. Turn the red bulb on and off. Notice which colors change, and how. Repeat with the green bulb, and then the blue bulb.

Try turning off the lights in sequence: first red, and then green, so only the blue light remains. What do you notice?

What's Going On?

We’re used to seeing the world around us illuminated by light in a full range of colors—and our eyes have evolved accordingly. A typical human eye has three types of cones, or color receptors, each sensitive to mostly reddish, greenish, or bluish light. When all three are stimulated simultaneously (as they are by the three bulbs used here), your visual system perceives “white,” rather than individual colors of light.

But colors seem to play tricks when you shift the light balance: A yellow fish may look green in one light, grey in another. The perceived color of any object you see comes from reflected light. For example, a red fish appears red because it absorbs all the many colors in white light except red, which it reflects back to your eyes. However, objects can only reflect back the light that’s available: A red object can’t look red if there’s no red light to reflect. Instead, it will simply look black.

Turning off the red bulb roughly approximates the effect of diving deep underwater. White light tends to lose its red component as it passes through water, leaving behind mostly blue and green light to illuminate the underwater environment. In this blue-green light, a red fish may look black, and a yellow fish may look green.

The altered availability of light in deep water has important consequences for aquatic organisms. Since very little red light is available, many aquatic animals have evolved to be completely incapable of detecting it, favoring detection of blue light instead.

Meanwhile, many prey organisms living at depths where blue light penetrates, but red does not, have evolved to be red. To their predators they appear nearly black, effectively disappearing in the dim light of the deep.  

Teaching Tips

This Snack is an excellent introduction to predator and prey adaptations in different environments. To demonstrate natural selection, have students try to “catch” as many fish in an image as they can in a given amount of time. Slide the image inside a page protector, and students can use whiteboard markers to mark the fish they can see.

Try this experiment under different lighting conditions, representing different ocean depths, and ask students to think about which fish are more likely to survive and reproduce in which environments.


NOAA How Does Depth Affect the Color of Marine Animals?