Light is the part of the electromagnetic spectrum visible to the human eye. It is influenced during transition from air to water and also in the water by reflection, refraction, scattering and absorption. These effects influences how far you can see, clarity, colors and apparent distances und water.
Basic knowledge for Open Water Diver* (OWD*)
Without a diving mask, under water you can see everything blurred, as the human eye is adapted to the refractive index of air. The human eye can not adjust to the higher refractive index of water. To be able to see clearly, one must create an air-filled space between eye and water. This air-filled space is located between the glasses of the diving mask and the eyes.
As a result, you can see clear, but the dimensions are no longer correct. Refraction occurs because the speed of light changes with the density of different media which causes a shift in the light path. In diving the light we see, passes through water, through the glass of our diving mask and finally though the air inside the diving mask until it reaches the eye. Our brain always assumes that light travels in a straight line. You therefore have the impression that an object under water appears to be about ⅓ larger and ¼ closer than it in reality is.
Light is scattered on the finest suspended particles in the water. The more suspended particles in the water, the lower the distance it can travel. The relative concentration of suspended particles is referred to as turbidity. Light scatters of particles as it travels through the water. This effect is called diffusion. Diffusion tends to make light spread more evenly which reduces or even eliminates shadows under water. It also may make things further away from you appear indistinct. As higher the turbidity as more diffusion, as less light penetrates and as lower will be the visibility under water.
By absorption light is converted into heat. The strenght of absorption is dependent on the wavelength of the light, i. e. its color, and on the distance the light travels through a media: The longest wavelengths, with the lowest energy, are absorbed first. Red is the first to be absorbed, followed by orange and yellow. The colors disappear underwater in the same order as they appear in the color spectrum. Even clear water at 5 m depth will have a noticeable loss of red. With increasing depth all objects tend to become more and more blue.
Color absoprtion is not only a function of depth, it is actually a function of the total distance light travels through water. If you are in very clear water only two meters deep, a red object right next to you will still appear red. If you swim 3 meters away from it, it will look brownish. This is because the light travels from the surface 2 meters down and than 3 meters to your eye making a total traveled distance of 5 meters.
For this reason, under water lights are usually used to add color back to subjects.
Knowledge for Experienced Diver** (ED**)
Refraction on separating layers
A refraction occurs not only on mask glass and the water surface, but also on separating layers with different refractive indices, such as between salt and fresh water or cold and warm water. As a result, everything appears blurred.
Visible light consists of wavelengths from 380 nm to 780 nm. The smaller the wavelength, the higher the energy of the light.
Different spectral colors are light with different wavelengths. Light with 450 nm, appears to the human eye as blue, 520 nm as green and 780 nm as red. Sunlight is a mixture of all colors with approximately the same intensity and appears white.
In water, light is more or less absorbed depending on its wavelength. The energy is converted into heat. Low-energy light, i. e. red light is absorbed already at shallow depths and is therefore only visible up to a depth of about 5 m. Higher energy light, i. e. blue light components penetrate much deeper and are still visible in 50 m. In the deep sea from about 200 m of depth almost all visible light has been absorbed and there is complete darkness. Therefore the spectrum changes continously with the depth of the water.
Knowledge for experts (DM***)
Light is refracted at the transition between air and water. From an incident angle higher than 42°, the light that is traveling from water towards the air is no longer passing through the surface, but is completely reflected back into the water. This is called total reflection.
If one looks at a calm water surface from below, one sees in a circular area everything above the surface. Outside this area, the light penetrating from the water upwards is totally reflected and overlays the light from above the surface. Therefore you can see a reflection of the underwater landscape that surrounds you.