When we open our eyes underwater, everything appears blurry and it is difficult to become used to that feeling. Moreover, in the case of seawater, due to salt, it is even more uncomfortable to keep our eyes open. However, with the mask on, that discomfort goes away and images appear clear, although objects appear to be larger (4/3 the size of the original) and closer (3/4 of the real distance). This is easily noticed when, for instance, we look at our hands.
When we put on a mask and go down underwater, we find that as we get deeper, light intensity decreases, some colors are lost and our visual range is smaller.
Let’s explain each of these phenomena.
The amount of light penetrating the water depends on how much is reaching the surface. Therefore, light intensity underwater on a cloudy day will be lower than on a sunny day.
There are two factors that influence the amount of light that bounces (reflection) or passes through the surface (and re– fracts): Firstly, the inclination of the sun’s rays according to its position on the horizon. At midday, when the rays are most perpendicular, more light passes through a surface. In contrast, at sunrise and sunset, rays are oblique and many get reflected.
Secondly, sea conditions also have an influence: The greater the swell, the less light passes through the water surface. It is basically the same effect as light passing through a smooth or rough piece of glass.
We must take into account the aspects commented above when choosing the right time for a dive or just to have an idea of the amount of light we are going to find.
In addition, light heats the water when passing through, losing energy and consequently decreasing its intensity.
This phenomenon is called absorption. The deeper we go, the more energy is absorbed and the less light is available. This decrease in intensity is fast: at a depth of 1 m, 60% has already been lost, at 10 m 86% and at 40 m 98.5 %. The absorption of light depends on the density of the water, its temperature, particles floating in it, etc. For this reason, each sea has different characteristics in regard to light which, moreover, may change every day.
The first 20 m is the most illuminated area and thus the place where an infinite number of underwater creatures live. As One Star Divers, we will focus on this range.
In addition to fluctuations in light intensity, there are also significant changes in colour within the underwater environment.
White light is composed of radiation of different wavelengths. Each of these wavelengths produces a sensation of different colour tones in our eyes, corresponding to one of the colours of the rainbow, from red to violet. Water does not absorb all radiation equally. The least absorbed chromatic shades are blue and green, which reach deeper. The most absorbed rays are red, which penetrates to very little depth and does not reach objects submerged at just a few meters. Consequently, a starfish that looks red on the surface, at more than 5 meters may look purple or almost black because light reaching that depth does not carry red.
When red and orange are lost, and yellow and violet are dimmed, green and blue predominate. These last two will be the backdrop colours for the underwater world.