Diving into the Secrets of Underwater Color Perception

From your early days of scuba diving, you may soon experience a myriad of brightly colorful fish especially when visiting tropical diving destinations around the globe. 
While diving deeper and deeper you will encounter and see different species.
For example, when reaching around 100 feet (30 meters) you may see the one shown in this picture below:

“Wow, it’s so beautiful,” someone may think to themselves.
However, while writing “‘Violet-line Anthias'” on the board, the guide shines an underwater torch light to illuminate the fish. 
What follows is truly astonishing!
It appears in the following bright colors below:

“Someone might think there’s something wrong with their eyes,” when looking up the first time. But it truly appeared blue, and then pink with the torch light shining on it!
The reason for this is the density of the water blocking ambient light.
Since the color of an object is the color of the light it reflects, less light means less color. Due to the density of the water, Red gets absorbed by 15′(1 M), Orange by 25′(8M), Yellow by 40′(12M), and Green by 75′(23M). For this reason, artificial lights are usually used to add color back to subjects. Remember, the distance in water for absorbing colors applies to both vertical and horizontal.

Light, Color, and the Nature of Water

Both on land and underwater, the source of light is sunlight (during daytime outdoors). Sunlight is a combination of various colored lights, creating white light.
Some of you might have experimented with a prism to separate light into its colors during elementary school.

Paradoxical as it may seem if we consider a red tomato, it absorbs colors other than red and reflects red light, which is why it appears red.

So why did the lighting on the fish cause a color difference when the light was on and when it was off? The explanation lies in water.
Water has the property of absorbing warm-colored light.
When light travels through water, it first absorbs warm-colored light like red, followed by colors like orange, and so on. Eventually, blue light remains. The longer the distance light travels through water, in other words, the deeper it goes, the warmer colors are absorbed, leaving only blue light.
When the light from the torchlight is shone on the fish, the fish reflects back pink light from its body, giving it a pink appearance to our eyes.
However, around 100 feet (30M) underwater in natural light, the pink light is absorbed by the surroundings, preventing reflection. This causes only the leftover blue light to bounce back, resulting in the blue appearance that we noticed.

Speaking of which, have you ever noticed that there are many redfish among the creatures living in the deep sea?

You might find it surprising that such striking colors fish manage to blend in within their deep-sea habitats. This is due to the absence of red light in those depths, allowing them to merge with the surrounding ocean hue.

As we’ve discussed, perceiving colors underwater is different from what we experience on land. So, if your goal is to be the most eye-catching even in the ocean, choose orange rather than a solid red!!