Why are the Oceans Blue?

In Chemistry, Nature, Physics by Mystifact

The Oceans are known to be beautifully blue. While this question may seem staggeringly obvious to answer, there’s more to it than you think. Why is it when you wash your hands, the water appears transparent and not blue; considering the water of the Oceans is blue? Why, in some cases, does the Ocean appear a different colour, i.e. almost green? All these questions will be answered within this article!

The Oceans

While some may think the Ocean is blue due to its water reflecting the sky, that is not entirely the case. To understand fully why it appears blue, we need to study water at a molecular level.

Constituents of White Light

As you may know, white light is composed of all wavelengths of visible colour [1]. Different wavelengths of light have different energies. In simpler terms, white light is a mixture of all colours of light combined. The image below demonstrates this with a simple prism. I will not explain the Physics of why this happens in this article, but this is valuable information for understanding why the Oceans are blue. Having learnt that white light is a mixture of all colour, we can now start to look at Water at a molecular level.

Figure 1: Image showing a prism splitting up white light into its constituent colours

The Chemistry of Water

The Oceans appears blue mainly to do with the absorption of light; allow me to explain. Remember when I said each colour in white light has a slightly different energy due to different wavelengths? Well, when white light hits a water molecule, these different wavelengths cause the molecule to vibrate differently [2]. It just so happens that light on the redder side of the spectrum have the right energies to get the water molecules wiggling more than the bluer side. What does this mean? This means redder light is absorbed a lot better than bluer light, so more blue light passes through water than red, yellow and green – hence why we see it as blue.

Why is a cup of water transparent?

While a cup of water may seem transparent to you, it actually lets slightly more blue light through than red; the problem is there is not enough water to visibly show the effect of the absorption of red for the water to appear blue. This also explains why the deeper you go down into the Ocean, the darker the colour of blue becomes. Since water still absorbs some blue light, going deeper into the Sea means its colour will appear a darker and darker shade of blue up until it is pitch black.

Can the Sea appear a different colour?

The answer is yes, but it depends on the situation. The Sea can appear greenish blue; this is due to certain algae on the sea bed of shallow water, that reflects green light. The Sea can also appear greyish blue due to dirtmixed in with the water that results in a change of tint of the Sea colour.

Under a Beautiful Sunset

My personal favourite is the appearance of the Sea under a sunset. Since a sunset is predominantly red and orange light, the lack of blue means most of the Sun’s light gets absorbed by the water, such that it looks almostblack. Next time you see the Sea, remember this article!

If you have any questions, leave them below and until next time, take care.

~ Mystifact

[1]: https://www.sciencelearn.org.nz/resources/47-colours-of-light
[2]: https://oceanservice.noaa.gov/facts/oceanblue.html

Please note; no copyright infringement is intended. All images used have been labelled for re-use on Google Images. If any artist or designer has any issues with any of the content used in this article, please don’t hesitate to contact me to correct the issue.

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