Physical Properties of Water

Density of Water

Water is a much denser medium than air.Balance In fact, it is about 800 times denser. Because of the increased density, a diver will encounter greater resistance while moving within and throughout the underwater environment than a person on dry land. New divers are routinely advised that they should streamline themselves and their equipment, and that they should move slowly and steadily while underwater in order to avoid overexertion. The level of exertion during a dive will have a direct impact upon the diver’s rate of air consumption and level of post-dive fatigue.

While increased exertion may be the most obvious issue related to the density of water, as will be demonstrated its density also influences its other properties, and thus has a more far-reaching impact upon the diver.

Physical Properties of Water

Refraction of Light in Water

The human eye “sees” by gathering and focusing the light emitted by or reflected from an object; together the cornea and internal lens (both at the front of the eye) focus that light upon the retina (at the rear of the eye) where it is converted into neural signals that then are interpreted by the brain.

Light is radiated electro-magnetic energy. Light rays bend, or refract, when they pass from one medium to another of different density. The amount of bending is determined by the refractive indices of the two media. The structure of the human eye also has its own refractive index, and the eye is adapted to sharply focus light rays entering the eye from air. When light rays enter the eye from water, the degree of refraction generally exceeds the eye’s focusing range, and thus the image appears blurry.Larger and Closer To restore sharp focus underwater, it is necessary to add an air space in front of the eye, such as that provided by the dive mask. The mask then creates additional refraction as light rays pass first from the water to the air space within the mask, before entering the eye. The refractive index of air is approximately 1.00, and for water it is about 1.33; accordingly, as light passes through the mask, the apparent size of an object is increased by about 33% within the diver’s overall field of view, making it appear that the object is about 25% closer to the diver.

Dome LensThe flat glass surface of the dive mask creates no significant refraction as light passes through it. Conversely a curved glass surface creates refraction, and effectively the curved glass now becomes an optical lens. Curved glass sometimes is used by underwater photographers, in the form of a clear domed-port over the lens of a camera in an underwater housing; the domed-port effectively widens the camera’s angle of view. It is important to note that the curved glass surface also changes focusing distance, and thus is usually impractical for use in a dive mask, because the change in focusing distance typically exceeds the normal focusing range of the eye.


Physical Properties of Water

Diffraction of Light in Water

Light rays normally travel in a straight line through a medium. Any minor obstacles encountered along the way will cause the light rays to be deflected and become scattered to some extent. This scattering, or divergence, is known as diffraction. It differs from refraction in that refraction is the uniform bending of light, while diffraction is the random divergence of light.

Diffraction routinely occurs above water when suspended moisture and dust particles, within the air, scatter the light. Naturally, the degree of diffraction increases with distance. The result is that the view of a distant above-water object can appear somewhat obscured and lacking in detail, and the brain has learned to associate this appearance with distance.

Being a denser medium than air, water is capable of supporting a far greater quantity of suspended particulates, and thus diffraction in water can be significantly greater than in air. In some cases diffraction can have a significant effect on the appearance of underwater objects more than just a few meters or yards away. Accordingly, the seemingly contradictory situation arises where refraction continues to make nearby objects appear to be closer to the diver than they actually are, while at the same time diffraction makes other objects appear to be even farther away. This phenomenon sometimes is referred to as visual reversal.


Physical Properties of Water

Color Loss in Water

Color Spectrum

The eye’s perception of color is dependent upon the combination of visible wavelengths of light emitted or reflected by an object. The apparent color of an object, illuminated by another source of light, is dependent first upon the wavelengths of light falling upon that object, and second upon the wavelengths of light that are reflected (and not absorbed) by that object. Each wavelength corresponds to a color in the visible spectrum of light; in descending order, from longest to shortest wavelength, the primary spectral colors are red, orange, yellow, green, blue, indigo, and violet (to assist the reader in remembering these spectral colors and their respective order, it is noted that the first letter of each color spells the fictitious name of “Roy G Biv”). The combined presence of all spectral colors is perceived as white, and the absence of all spectral colors is perceived as black.

In terms of human vision, sunlight is basically white in color. As sunlight passes through the denser medium of water, some wavelengths of light are absorbed by the water; in effect, the corresponding colors are removed from the illuminating light, and thus no longer can be reflected by an object. The longest wavelengths (red) are absorbed the quickest by water, and this occurs at a relatively shallow depth. Additional wavelengths are absorbed when depth increases, as sunlight passes through progressively greater distances of water. Eventually, at the deepest sport diving depths, objects are illuminated only by the shortest wavelengths of light and thus largely appear as varying shades of grey.

Artificial LightOf course, while absorbing certain wavelengths, water also can add its own tint to the light which passes through it. Tropical saltwater tends to be blue, and more temperate saltwater tends to be green because of the nutrients typically present in the water. Silt and other suspended particulates, as may be found in both saltwater and freshwater, also can turn the water green or brown. These additional tints will contribute to the overall appearance of objects underwater.

When a diver illuminates an underwater object with a handheld dive light, the true colors of that object become more readily apparent. This occurs simply because the illuminating light is now passing through a much shorter distance of water. Underwater photographers often employ an electronic photo-flash, not only to illuminate a dark subject but also to more accurately depict its natural colors.

Physical Properties of Water

Transmission of Sound in Water

Sound is a series of vibrating waves of energy transmitted through a solid, liquid or gas. These vibrations are gathered by the ear and transformed into neural signals, which then are interpreted by the brain. The speed of sound depends upon the medium through which it passes, and sound tends to travel faster in denser mediums. The precise speed of sound in water depends upon pressure, temperature, and salinity, but in general sound travels about four times faster through water than air.

The brain interprets the direction of sound based upon the minuscule difference in time between the arrival of that sound at one ear versus the other. This tends to work well above water, with sound being transmitted through the air. Underwater, however, the increased speed of sound makes it difficult to determine direction because the difference in time between that sound reaching each ear is significantly reduced.

Sound Speed

The density of water, combined with the non-compressibility of all fluids in general, makes it an excellent medium for the transmission of sound; and thus sound will travel over a greater distance in water than air before it dissipates. Unlike light, however, sound usually does not transmit well from one medium to another. Instead sound typically is reflected and dissipated at the point where two different media meet. Sound that originates above water usually cannot be heard underwater, and sound that originates underwater usually cannot be heard above water. Sound also can be reflected and dissipated by a halocline (where saltwater meets freshwater) or a thermocline (where warmer water meets cooler water), due to the differences of the media at these points.

Physical Properties of Water

Thermal Conductivity of Water

Wetsuits 1

Heat is a form of radiant energy that can move through solids, liquids and gases. The thermal conductivity of any substance depends upon a number of variables including its density and overall structure. Water is a far more efficient thermal conductor than air. In fact, the relative thermal conductivity factor for water is about 6.0, while for air it is only about 0.025; thus water will whisk heat away from a diver’s body approximately 25 times faster than air. While an air temperature of 27°C or 80°F typically is perceived as warm, the same water temperature of 27°C or 80°F usually will be perceived as cool. After some length of time in the water, when the water temperature is anything less than a diver’s core body temperature of about 37°C or 98.6°F, the diver eventually will become chilled. Accordingly, knowledgeable divers usually opt for some form of exposure protection even in relatively warm water.


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Chapter 4 Quiz

Diving Physics: Part 2

Topics Covered in this Chapter:

  • Archimedes’ Principle
    • Buoyancy
    • How this Principle Relates to Diving
    • Seawater versus Freshwater
    • Buoyancy Calculations
  • Review Questions


Archimedes’ Principle


Archimedes of Syracuse (Greek mathematician, astronomer and inventor, circa 287-212 BC) is regarded as one of the leading scientists in classical antiquity. His studies and writings were quite varied and extensive, encompassing a broad range of subjects. Most importantly, in the context of this text, he discovered the principle known in physics as Archimedes’ Principle. This principle states that when an object is immersed in a fluid, it experiences a buoyant force equal to the weight of the displaced fluid.

Archimedes’ Principle

How this Principle Relates to Diving


Thanks to Archimedes, divers now know why some items float while others sink. When an object is placed in water, it displaces an amount of water equal to its own volume. Naturally, the object and the displaced water each have a certain amount of weight. If the weight of the object is less than the weight of the displaced water, the object will float; in this situation, the object is positively buoyant. Conversely, if the object weighs more than the displaced water, the object will sink; in this case the object is negatively buoyant. If the object weighs exactly the same as the displaced water, it will neither float nor sink, and instead it will simply hover in mid-water; here the object is neutrally buoyant.

As an example, consider a golf ball and a table-tennis ball. Each is about the same size and, as such, each displaces the same volume of water. However, because of its solid core, the golf ball weighs considerably more than the hollow table-tennis ball. When placed into water, the table-tennis ball naturally will be more buoyant than the golf ball. In fact the table-tennis ball will float, and the golf ball will sink.

It is important to note that there are degrees of positive and negative buoyancy. If the immersed object weighs only slightly less than the displaced water, that object will slowly ascend to the surface of the water; but when the object weighs significantly less than the displaced water, it will ascend more quickly and forcefully. If an object weighs only slightly more than the displaced water, it will slowly descend; but if the object weighs significantly more, it will descend more quickly and forcefully. It is equally important to note that neutral buoyancy is an exact point, either the object weighs precisely the same as the displaced water, or it doesn’t.

Divers use three items to control their own buoyancy: lead weights, a BCD (buoyancy compensator device), and lung volume.

WeightDivers come in all shapes and sizes. Some persons are positively buoyant, while others are negatively buoyant. However, after donning an appropriate exposure protection suit for the environment in which they are diving, sport divers usually find they are somewhat positively buoyant. To compensate for this inherent buoyancy, the diver will require some amount of lead weight in order to descend below the surface of the water. Lead is ideal for this purpose; it is an extremely dense material that displaces relatively little water in comparison to its weight.

While in the water, a diver uses the BCD to control buoyancy. When air is added to the BCD, its inflatable bladder expands and thus displaces an increased volume of water; as a result the diver becomes more buoyant. When air is vented from the BCD, its inflatable bladder compresses and thus displaces a decreased volume of water; as a result the diver becomes less buoyant.

As many experienced divers already know, lung volume can be used to fine tune buoyancy control. When a diver inhales, his or her chest expands accordingly and an increased volume of water is displaced; effectively the diver becomes more buoyant. When the diver exhales, the chest compresses and a decreased volume of water is displaced; effectively the diver becomes less buoyant. By simply altering the lung volume either by inhaling a little more deeply than normal or exhaling a little more fully, the diver is able to make subtle changes in buoyancy on a temporary basis, without having to adjust the inflation of the BCD.