When we look at large bodies of water like oceans, lakes, or rivers, it seems blue. Water even in swimming pools also looks bluish. The blue color of water is actually due to the way it interacts with light. After reading this article, you will learn why water looks blue, what factors influence water coloration, why are glaciers blue in color, and why icebergs, snow, and milk look white.

Why Is Water Blue In Color?

The water looks blue because of the selective absorption and scattering of sunlight. Water in small quantities like in a glass, jug, or bucket seems colorless to human eyes. However, water is not colorless, it gives a light blue tint that can be observed when looking through a long water column.

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The question arises why does water appear specifically blue? The reason is that water absorbs more light of long wavelengths like red and yellow from the sunlight, leaving light of shorter wavelengths like blue and green that scatter on the surface. That is why water looks blue or blue-green.

Water molecules can efficiently absorb light of longer wavelengths. Sunlight is made up of different colors, a spectrum ranging from red and yellow of longer wavelengths to blue and violet of shorter wavelengths. When sunlight hits the water surface, it absorbs red and yellow light of longer wavelengths, leaving behind violet, blue, and green.

So, when we look at a large body of water, such as lakes and oceans, it seems blue because blue light scattered on the water’s surface reflects to our eyes. The different shades of blue water depend on the depth of water and the particles suspended in it.

Let’s discuss some basic concepts to understand why water looks blue:

The Nature Of Light And Human Perception Of Colors

The part of light that human eyes can perceive is known as visible light. Light travels in the form of waves, which have different lengths known as wavelengths. The wavelength range of visible light is from 380 to 750 nm.

The red, orange, and yellow colors in the visible light have longer wavelengths while violet, blue, and green colors have shorter wavelengths.

When light hits an object, it absorbs some colors while the remaining color scatters and reflects to our eyes. For example, a strawberry looks red because it reflects red light. Special cells in our eyes (known as cones) detect these different colors and send signals to our brain. Brain then interprets these signals as colors. In this way, we perceive different colors around us.

The Absorption Spectrum Of Water

The absorption spectrum of water is a way to see which colors water takes in or absorbs from light. Water absorbs colors of the longer wavelengths of sunlight, which are red, orange, and yellow. It means that water does not let these colors pass through easily.

So when sunlight shines on water, the red, orange, and yellow colors are absorbed and mostly blue, blue-green, or cyan colors (shorter wavelengths) are left behind. That is why water usually looks blue or green to us.

Rayleigh Scattering: A Key Phenomenon

Rayleigh scattering is a phenomenon of light scattering in different directions by particles in its path smaller than the wavelength of the light. No loss of energy or change in wavelength occurs in this phenomenon.

This phenomenon is responsible for making things appear blue, whether sky or water.

Sky

When sunlight enters the Earth’s atmosphere, molecules of nitrogen and oxygen (which are much smaller than the wavelengths of visible light) scatter the shorter wavelengths (violet and blue). That is the reason for the blue color of the sky.

At sunset and sunrise, the sunlight has to pass through more of the Earth’s atmosphere, leading to even more scattering, and causing the shorter wavelengths to scatter away. The remaining longer wavelengths (red and orange) color the sky at dusk and dawn.

Water

Water molecules are also much smaller than the wavelengths of visible light. But they can absorb longer wavelengths (red, orange, yellow). So When visible light hits water, it absorbs longer wavelengths, and the remaining shorter wavelengths (blue, green) scatter on its surface. That is why, oceans and lakes appear blue and green to our eyes.

Molecular Structure Of Water And Light Interaction

One oxygen and two hydrogen atoms bond together and form a water molecule. This molecular structure plays a major role in how it interacts with light.

When water molecules absorb energy from light, a pair of electrons on the oxygen atoms become excited. The energy levels of these electrons allow them to absorb longer wavelengths (red, orange, yellow) more effectively.

The specific vibrational modes of water molecules (symmetric, bending, and asymmetric), and their fundamental frequencies allow them to absorb longer wavelengths of light more readily.

When light hits the water’s surface, some of it is reflected and some enters the water and is refracted (bends). This refraction interacts with objects beneath the surface and affects the clarity and color of water.

Factors Influencing Water Coloration

The following are the main factors that influence water coloration:

Depth:

Deeper water appears blue because longer wavelengths are more readily absorbed than shorter wavelengths when light travels through water. The penetration of light decreases as the water deepens, so the shorter wavelengths are scattered more on the surface, and water seems darker in color. That is why the oceans seem so dark as to be almost black in the Earth’s images taken from space.

Impurities and suspended particles

Impurities and suspended particles can also affect water coloration. For example, dissolved organic matter or tannins from decomposing vegetation can give water a brownish tint. While algae and suspended sediments can give it a murky or greenish appearance.

Algal blooms

The excessive growth of algae (algal blooms) can cause the discoloration of water. Some species of algae produce pigments that can give a green, red, or brown color to water, depending on the species of algae.

Minerals and dissolved substances

Certain minerals and dissolved substances in water can contribute to its color. For example, a high concentration of iron gives a reddish-brown color to water, while a high concentration of lime (calcium oxide and hydroxide) gives a cyan hue.

Color of the sky and surrounding landscape

The sky color and surrounding landscape can also affect water coloration. This is special for clear or shallow water bodies. For example, if you look at water in an indoor swimming pool from above, it will seem cyan, and the same water in a small bucket will appear slightly cyan or almost colorless. But if you observe the same water from a close range, it will seem colorless.

Why Do Icebergs And Snow Appear White?

Snow looks white because it has a large number of tiny air bubbles that reflect all wavelengths of visible light.

Icebergs look white because they have compressed snow on their surface. They also have crystal edges that together with air bubbles of the surface snow reflect all wavelengths of visible light.

Are Glaciers Blue In Color?

Glaciers look blue because they also absorb part of the light with longer wavelengths (red, orange, yellow) and scatter the shorter wavelengths (violet, blue, green).

The edges and surface ice of glaciers look white because of air bubbles. But as the glacier compacts, the air bubbles squeeze out due to the increasing pressure. Similar to large bodies of water that look blue or cyan, glaciers also look blue or cyan.

Why Is Milk White?

Milk is white because of its lipid or fat content. Lipid molecules are much larger than the wavelength of light, so they reflect all wavelengths of visible light. If all the fats from milk are removed, it will not be white. That is why skim milk has a slight bluish color.