When we think of oceans and lakes, we think of sparkling blue waters. However, upon closer investigation, we see that water is clear. The reason why water is clear is that it is made up entirely of oxygen and hydrogen. Because both of these elements are gases, their electrons are unable to absorb or reflect visible light. In fact, water refracts or changes the direction of light. For example, when a T-shirt is soaked with water, it refracts away light, making the object appear darker. This is why absorbed water darkens material, and why water is clear.
Why Water Is Clear If Ocean and Lake Water Looks Colored
We know how and why water is clear, so it probably doesn’t make immediate sense to us that while a small amount of water is clear, lakes and oceans appear to be blue. The reason for this is that water does not absorb much light, but when it does absorb light, it absorbs red, orange and yellow light. As a result, it reflects back the shorter blue wavelengths to observers.
Why Water Is Clear: Misleading Opacity
Large bodies of water do not always appear blue. Many rivers can appear brown, green or even gray. These appearances can be explained by the number of dissolved or suspended particles present in water, and the depth of the water. Both particles and water depth influence how light is reflected or refracted to the observer. Color variants arise depending on the following circumstances:
- Gray water is generally water that has been stained by runoff from parking lots, buildings and roads in urban areas.
- Brown water is colored by dissolved organic materials like plants and animals. It is usually found in forests and wetlands.
- Green water is usually stained by suspended particles of living materials, like algae or other microscopic plants.
Why Water Is Clear: Checking for Clarity
If water is clear, there is a much better chance that it is clean. This is why we must check whether water is turbid or hazy. We can check water clarify with a Secchi Disk. This instrument is a black-and-white circular plastic plate that can be lowered into water. To use a Secchi Disk, first lower it into the water. Stop lowering it when you can’t see it anymore. Next, note the depth (in meters) off of the calibrated line. Then raise the disk back up to where it reappears, again noting the depth off of the calibrated line. Finally, add these two noted depths and divide them by two. This final value can help you gauge water’s clarity. Be sure to compare this value on a weekly basis with measurements at the same lake.