Water appears colorless and transparent in small quantities, such as in a glass. However, when observed in large bodies, such as oceans and lakes, water often appears blue. This is due to the way light interacts with water molecules. There are a few key factors that contribute to water’s blue color.
Why Does Water Appear Blue?
There are two primary reasons why large bodies of water appear blue:
- The absorption of long wavelengths of light by water molecules
- The scattering of short wavelengths of light
Water molecules absorb light in the red end of the visible light spectrum. As sunlight enters water, the longer red wavelengths are preferentially absorbed, so less red light is reflected back out. This leaves shorter wavelengths like blue and green to be reflected back out. This gives large bodies of water a predominant blue color.
In addition, the water molecules scatter blue wavelengths more than other colors. This scattering reinforces the blue color that is reflected. The scattering effect is enhanced in lakes and oceans, where there are many trillions of water molecules for light to interact with.
Absorption of Long Wavelengths
The first factor that contributes to water’s blue appearance is preferential absorption of long wavelengths of light. Visible light from the sun consists of a spectrum of wavelengths, ranging from short high-energy violet/blue to long lower-energy red. Water molecules absorb light photons and the energy causes the molecules to vibrate and rotate. This molecular motion accounts for much of water’s heat capacity.
Water molecules absorb infrared, red, orange, yellow, and green wavelengths of sunlight. However, the absorbance is not equal across the spectrum. The longer wavelength colors toward the red end are more readily absorbed than short wavelength blue and violet light. This selective absorption removes more red light, leaving the shorter blue wavelengths to be reflected back out. This gives water its blue color.
Scattering of Short Wavelengths
The second factor is that water scatters blue light more than other colors. When light enters water, the water molecules scatter the light. Shorter wavelengths of light are scattered more than longer wavelengths. Blue wavelengths are scattered most strongly, while red wavelengths are scattered least. This scattering reinforces the blue color that is reflected back out.
scattering and absorption explain why when boats travel to the open ocean or lakes expand in size, these bodies of water turn very deep blue. In small amounts of water, scattered light doesn’t predominate, but as the number of water molecules increases, the scattering effect becomes pronounced, enhancing the blue color.
Variations in Color
While water generally appears blue, the exact color can vary somewhat based on several factors:
- Viewing angle – The blue color is most intense when viewing water straight on. At shallow angles, more light is reflected, making the water appear lighter.
- Contents in water – Dissolved and suspended material can absorb and scatter light, altering the shades of blue.
- Surface conditions – Choppy water appears darker blue as more light is absorbed, while calm waters appear lighter blue.
- Time of day – Overhead midday sun scatters light evenly, while lower afternoon sun produces darker blues.
- Location – Different geographical areas have differing minerals and organisms that can also affect color.
While subtle variations in shade occur, the main blue color stems from the intrinsic absorption and scattering properties of water molecules.
Other Color Perceptions
In some situations, large bodies of water can appear green, grey, or even black:
- Green water – Algae blooms or suspended sediments can impart a greenish cast.
- Grey/black water – Heavily overcast skies or oblique sun angles limit blue light scattering, causing water to appear darker gray or black.
However, these effects are secondary to water’s fundamental blueness from light absorption and scattering. Only in small quantities or under unique conditions does water not elicit a blue color.
The Clear Blue Ocean
The ocean provides the quintessential example of water’s blue color. Ocean water contains trillions of water molecules that absorb long red wavelengths of sunlight and scatter short blue wavelengths. This produces a deep, clear blue appearance.
Coastal waters can be less blue and take on green, grey, or brown hues due to sediments from rivers and runoff. But in the open ocean, the water becomes a deep dark blue. This is especially true in the clearest parts of the ocean, such as the Sargasso Sea in the North Atlantic Ocean, which has very low nutrient and algae levels.
Fresh Water Lakes and Streams
Freshwater lakes and streams also demonstrate water’s blue color, though it may be less pronounced than the ocean. Many lakes have greater levels of dissolved and suspended material than seawater, which can mute the blue color somewhat. However, in clear lakes and streams with minimal sediments and algae, the water exhibits a distinct blue cast from the selective absorption and scattering of sunlight.
Water’s Unique Blue Absorption
Interestingly, water’s blue color cannot be replicated by any other liquid. Other liquids and transparent materials like glass absorb light evenly across the visible spectrum. They do not preferentially absorb reddish wavelengths and scatter blue light like water does. This unique molecular behavior is what makes water intrinsically appear blue.
Practical Applications
The fact that water absorbs long wavelength light has some useful applications. Solar water disinfection takes advantage of water’s absorption of ultraviolet and infrared light to kill microbiological contaminants in water, helping make it safe to drink and use.
Understanding the selective absorption and scattering properties of water also allows oceanographers and limnologists to use blue color as an indicator of water composition and clarity. The more blue the water, the less suspended material it contains.
Conclusion
While pure water is colorless, the large bodies of water that cover our planet appear blue. This blue color arises from the intrinsic absorption and scattering properties of water molecules interacting with sunlight. The preferential absorption of reddish wavelengths coupled with greater scattering of blue light gives water its distinctive color that is vital to life on Earth.
| Color | Wavelength Range | Absorption/Scattering in Water |
|---|---|---|
| Violet | 380-450 nm | High scattering |
| Blue | 450-495 nm | Very high scattering |
| Green | 495-570 nm | Moderate scattering |
| Yellow | 570-590 nm | Low scattering, moderate absorption |
| Orange | 590-620 nm | Very low scattering, high absorption |
| Red | 620-750 nm | Minimal scattering, very high absorption |
This table summarizes how different visible wavelengths of light are absorbed and scattered in water. Shorter violet and blue wavelengths are scattered the most, while longer orange and red wavelengths are preferentially absorbed, contributing to water’s blue color.