Light and Vision Changes
The human eye is adapted for vision in the air way. The light rays from the object travel through the air, pass through the pupil, cross the lens (half watery) and form a focused image on the retina, hence a clear image to the brain through the optic nerve.
When immersed in water, the light rays from the object travel through the aquatic environment penetrating through the pupil, cross the lens (half aqueous) and will form the image behind the retina (blurred), so the image taken to the brain through the optic nerve is diffuse.
As written in Module 2 – Scuba Diving Equipment, the function of the mask is to avoid this inconvenience by placing a layer of air between the aquatic environment and the eye. Thus, the light rays, when crossing this aerial medium and the crystalline, will form an image focused on the retina, which, in this way, is clear.
However, the light rays are diverted from the water passage (where the object is) to the aerial environment (inside the mask) and the object appears larger and closer to the diver.
Another phenomenon that occurs is the absorption of light. The light rays when penetrating the water suffer a rapid loss of their luminous intensity. This loss is due not only to water absorption, but also to diffusion caused by particles that are in suspension. At sea, on average, light is about 40% to -1 meter, 14% to -10 meters, 7% to -20 meters and only from 1.5% to -40 meters.
At the same time as this progressive extinction, another phenomenon is produced, which modifies the spectral composition of light and causes a change in colors. In fact water behaves like a filter that progressively absorbs the radiation that makes up the solar spectrum. First the longer wavelengths (infrared, red, etc.) are absorbed and finally the shorter wavelengths (green, blue) are absorbed.