In the cover photo for this blog post, you saw a scuba diver photographing an object below them; everything in the photo is black and white. We often take this kind of photo for granted--after all, it may be edited--but there actually is a depth under water at which you or I would see everything in black and white. Why is this?
We all know that water absorbs and reflects light. Everyone has experienced light reflecting off the water's surface, and often, a person will learn as a child that the ocean has its "pitch-black, crushing depths" at which no human being can go. Interestingly, however, it doesn't take nearly as much depth for color to disappear. Here's a little lesson on color!
White light is made up of different "wavelengths" or frequencies of energy that determine what we can or can't see. Some wavelengths are visible to us: the colors, while other wavelengths are invisible to us. Some invisible wavelengths can be of higher and more energetic frequencies than visible light, and include X-rays and ultraviolet light, producing effects we know as useful in the case of science and medicine or harmful in the case of sunburns. The lower and less energetic frequencies include infrared, microwave, and radio, all of which we've known an immense list of uses for. All of the visible colors are mixed in white light, which is apparent to us when sunlight bounces off of water in the atmosphere to create a rainbow, or when white light hits a prism and is split into its different visible wavelengths.
Different chemicals and compounds absorb or reflect color in different ways--plants are green, for example, because they absorb wavelengths such as reds and yellows from sunlight and don't absorb, and consequently reflect, green wavelengths of color away from themselves (and into the eyes of outside observers like ourselves)! Water, as a unique chemical in many respects, is no exception in that it has its own unique "absorption spectrum," or pattern of light absorbance (this also tells us about its light transmittance, or what kinds of light it reflects most). We see the ocean as blue or blue-green for this very reason!
You'll already have learned enough from this blog post to have a hint at what's coming next. In water's unique absorption spectrum, water absorbs some colors faster than others. See the graph below which shows exactly how!
"Light Penetration Spectrum in Water", Image from Wikimedia Commons https://commons.wikimedia.org/wiki/File:Light_Penetration_Spectrum_in_Water_01.png by user Tomemorris, licensed use under Creative Commons CC BY-SA 4.0, https://creativecommons.org/licenses/by-sa/4.0/deed.en.
The main source of white light you'll find anywhere in the ocean comes from above it--the sun! As sunlight travels through sea water, the water absorbs it and turns it into heat, or reflects it back towards the surface--and so, less and less light (of any wavelength) is available to deeper depths. This has readily noticeable effects very quickly!
Who and what does all this science of color and water absorbance affect? Water absorbs all red wavelengths of light by roughly 10 meters, or about 33 feet. Most fish in deeper parts of the ocean with significant color to their bodies are red, which actually is a result of evolution--because that is the first color to appear gray or black to predators at depths below 10m/33 ft, those fish have always been destined to survive better than prey of other colors. After red's disappearance in the depths of the sea, yellow is the next color to be absorbed at about 23 m/75 ft, which is followed by green at about 35m/115 ft, and finally, blue at about 40m/130 ft (of course, I'm leaving out some colors to simplify the explanation). This is why at deeper recreational diving depths (anywhere from about 23 m/75 ft to 40 m/130 ft), everything appears to be in shades of blue and green to scuba divers. How do they compensate? They bring flashlights to re-introduce white light into their surroundings, the color in which can once again be selectively absorbed and reflected by different objects such as fish, corals, and each other's dive equipment to show color! The disappearance of color with depth is also why the best coral reefs grow almost exclusively in about 20m (60 ft) of depth! Corals depend on good sunlight with a lot of color, the best of which is found in those shallow depths.
Resources:
Comments