Introduction

Why would ocean water appear blue, but when you're at home and you fill a glass with water it doesn't appear blue, it looks clear instead?

Shades

Water from the ocean doesn't look blue all the time. Sometimes it looks to be a darker shade of blue, green, or clear. For example, at the beach when it’s cloudy the water looks really clear. But at different times of the day, it can appear to look different colors. Like at sunrise it can look reddish or different colors.

How water absorbs light

When the light hits the water in the ocean it absorbs the light rays and reflects the color back to the sky. About all the light that enters the sea is absorbed except if it is close to shore.

One answer to why the ocean is blue is because of the way water absorbs light particles. In the ocean, water scatters light. Sun rays are not all absorbed in the same way. Water molecules (the tiny particles that everything is made of) mainly absorb the longer wavelengths of light, such as red, orange, yellow, and green wavelengths. But blue (and also violet) is a shorter wavelength, so it gets reflected back to our eyes. This is why the ocean water usually looks blue.

It doesn't reflect off the sky

Another reason the water appears blue involves reflection although it’s not because the ocean reflects off the sky. Ocean water doesn't reflect off the sky. The water creates the shades by itself.

Photons and why things have color?

Light is made of a material known as photons. Sunlight rays contain all the colors of the rainbow. These colors are red, orange, yellow, green, blue, indigo, and violet.

To understand why the ocean is blue it’s helpful to know why things have color. Maybe you've heard that white light is filled with photons from all the colors in the rainbow. But not all things are white.

For example, an apple is red. So the apple absorbs all of the colors except for red. The red light is not absorbed by the apple. Instead it reflects back to your eyes so the apple appears red.

So, if the ocean appears blue, it absorbs all the colors of light except blue. The blue light reflects back to your eyes so the ocean appears blue. And that's why the ocean looks blue, but when you fill a cup with water it doesn't look blue.