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Why is the Ocean Blue? What is blue exactly?

How depth determines colour

The deep blue ocean. The rapture it has over us is mesmerizing and has captivated divers since man made the first rudimentary snorkel out of a hollow reed.  But what actually makes the blue, well – blue?

As a diver you may have noticed that the colour of the ocean varies – and this it largely due the waters ability to absorb colours. The more water = the more absorption, and this is why depth determines colour.

The text book definition of blue is the colour in-between violet and green on the visible spectrum.  Included in this definition are also the hues of blue – indigo, ultramarine, cyan, turquoise, teal and aquamarine.

The shade of blue that we experience underwater is dependent on many things – sunlight, algae, plankton, pollutants, sediments, freshwater and most important of all; depth. As we descend deeper and deeper from the surface of the water the bluer the water becomes around us. On the surface it is easier – the sea is blue because it reflects the sky. But we are divers – and it is the rapture of the deep that calls out to us.

Let’s talk a little about absorption.  Water has a huge capacity for the absorption of light – or more specifically, the different wavelengths of light. Water molecules vibrate, just as all things do that are made out of atoms. The vibration of water molecules causes absorption in the infrared part of the spectrum; 698nm to be specific.

Red has a wavelength of 700nm, which is just on the borderline of the visible spectrum. This is why red is the first colour to disappear. Next to go is orange, then yellow and so on – working its way through the colours of the rainbow one by one, until only green, blue and violet can be seen. These colours have a wavelength range of between 650-400nm which is why these colours travel further into the deep than any other colour. There is simply less abortion at this end of the spectrum.

In shallow water all of the colours are present, but only the lower ranges of the spectrum will follow you into the deep, blue ocean. Now we understand why it is called exactly that.

Evolution has allowed various undersea critters to utilize this colour absorption capacity to their advantage. One example of this is Sharks. Animals don’t have much use for colour and are often the same colour as the water that surrounds them.  Blues and greys are almost invisible from above or sideways, which is why they make the perfect camouflage. Sharks spend most of their time in mid water. Predators below them would see their silhouette – regardless of their colour, so they have evolved to be two-toned: dark on the top and light on the bottom. They literally blend in with the surrounding light. I think that’s pretty awesome, don’t you?

Other deeper dwelling fish have developed one step even further. They fight light with light. By producing their own radiant shine in the organs of their underbellies and keeping a dark colour on top they become invisible both from above and below.

You can even take this one step further, and use the colour red as your camouflage. This makes you invisible! Light in the upper ranges in the spectrum travel the shortest distance – a cloaking device so to speak. Red cannot even be perceived as red underwater – this is why blood looks so weird when you mistakenly cut yourself of a sharp bit of coral.

So there you have it. Now you can experience the blue of your next dive in a whole new light, with full understanding, and allow the emersion of your dive to envelop you totally. Enjoy the different shades of blue, for it is truly one of the most beautiful, gradual transitions that we mere humans can hope to experience while on this little blue planet.

The deep diver speciality course is perfect for those of you who want to experience this. Certification takes just four dives and allows you to go to the recreational depth limit of SCUBA ; 40 meters monkeys!!

Many sharks are two-toned. Light on the bottom and dark on the top; allowing them to blend in to their surroundings.


Happy diving monkeys!!

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