Miscellaneous Physical Phenomena
Since water is a medium of much greater density than air, several other physical phenomena occur, mainly connected with heat, light, and sound.
Underwater Vision
A ray of light passing from a medium of one density into a medium of a different density at right angles to the surface of the second medium continues in a straight line called the ‘normal’. If, however, the ray strikes the surface at an angle other than a right angle, the ray is bent at the surface and this bending is called refraction.
The degree of bending or refraction depends upon the difference between the densities of the two media. A light ray passing from the air into the denser medium of water is bent toward the normal but when passing from water to the less dense medium of air the ray is bent away from the normal as shown below:
The diver is mostly concerned with the effect of the bending of light rays passing from the surrounding water to the less dense medium of the air in their facemask. This refraction will give a false impression of the position of underwater objects, which will appear to be larger than their true size and only about three-quarters of their true distance away.
In Fig the diver is looking directly at object ‘A’ and the ray of light from the center of the object travels along the normal line through the diver’s visor without refraction. The light rays from the top and bottom of the object, however, both strike the visor at an angle and are bent in the air of the facemask along the dotted lines, and the diver sees these points along the dotted lines at ‘T’ and ‘B’. ‘A’ will thus appear larger and, in consequence, nearer.
In above Fig shows how refraction limits the diver’s vision underwater. When a diver stands in the air, there is no difference between the densities of the media inside and outside their helmet and consequently, no refraction, and their field of vision is shown by the full lines. Underwater, however, owing to refraction, only light rays between the dotted lines will reach their eye. A further result of refraction is the considerable distortion of objects viewed through the visor at an oblique angle, and, outside an angle of 48.6° from the normal, objects will not be observed at all because of total internal reflection.
Conclusion:
underwater phenomena involve the refraction of light, affecting vision and perception. Refraction causes objects to appear larger and closer underwater. Distortion and limited vision occur due to refraction, particularly beyond a 48.6° angle from the normal.
