Cited Passage
Physics of Diving 2-15
water or by introducing a small amount of water inside a
full-face mask. It can be dangerous to use gum or candy to
reduce dryness while diving. Do not remove your mouth-
piece in seawater or freshwater that may be polluted. 2.9.1 Humidity
Water vapor (a gas) behaves in accordance with the
gas laws. However, because water vapor condenses at tem-
peratures we are likely to encounter while diving, the
effects of humidity are important considerations. 2.9.2 Condensation in Breathing Hoses or Mask
Exhaled gas contains moisture that may condense in
breathing hoses of a rebreather or in your mask. This water
is easily blown out through the exhaust valve and, in gen-
eral, presents no problem. However, in very cold water,
this condensation may freeze, disrupting normal function-
ing of a scuba regulator. The dive should be aborted if such
a condition occurs. 2.9.3 Fogging of the Mask
Masks become fogged because of the moisture in
exhaled breath, or because of evaporation through facial
skin. This fogging can be prevented by moistening the face
plate with saliva, liquid soap, or commercial anti-fogging
products. Exhalation through the mouth, instead of the
nose, will reduce face mask fogging. 2.10 LIGHT
The sense of sight allows perception of electromagnet-
ic energy (light). Human beings can perceive only the very
narrow range of wave lengths from 380 to 800 nanometers
(see Chapter 3). Eyes function by collecting light that is
emitted or reflected by an object. Some light is absorbed by
the object, making the object appear colored. The energy
waves that are received by the eye are turned to electrical
impulses in nerves and sent to the brain via the optic nerve. The brain interprets the signals and we “see.”
Under water, the eyes continue to function by collect-
ing light reflected off objects, but the light itself changes. Water slows the speed at which light travels. As light
enters or leaves water, this change in speed causes light
rays to bend, or refract (see Figure 2.9 and 2.10). That is
why a pencil in a glass of water looks bent. Seen through a
diving mask, refraction affects close vision, creating distor-
tions that affect eye-hand coordination and the ability to
grasp objects under water. By placing a pocket of air (i.e., a facemask) between the
water and the eyes, the light rays are refracted twice
_
once
when they enter the air from the water and again as they
enter the eyes; a clearer image is now focused on the reti-
na. Due to imperfect correction, however, the retinal image
is larger. Objects may now appear approximately 25% larg-
er because of the larger-than-normal retinal image. The visual distortions caused by the mask vary consid-
erably with the viewing distance. For example, at distances
of less than four feet (1.2 m), objects appear closer than
they actually are. However, overestimation occurs at dis-
tances greater than four feet, and this degree of error
increases in turbid or muddy water. Other perceptual dis-
tortions are also apparent. Stationary objects appear to
move when the head is turned from side to side. Turbidity is another factor affecting underwater visibili-
ty. Turbidity refers to the clarity of the water, and depends on
the quantity of particulates in suspension. Muddy water is
more turbid than clear water. Turbidity can cause overesti-
mation of the distance of an underwater object. It is important to remember that underwater distance per-
ception is very likely to be inaccurate and that errors of both
underestimation and overestimation may occur.
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