Diving physics, or the physics of underwater diving, is the basic aspects of physics which describe the effects of the underwater environment on the underwater diver and their equipment, and the effects of blending, compressing, and storing breathing gas mixtures, and supplying them for use at ambient pressure. These effects are mostly consequences of immersion in water, the hydrostatic pressure of depth and the effects of pressure and temperature on breathing gases. An understanding of the physics behind is useful when considering the physiological effects of diving, breathing gas planning and management, diver buoyancy control<!-- disambiguation page intended --> and trim, and the hazards and risks of diving.
Changes in density of breathing gas affect the ability of the diver to breathe effectively, and variations in partial pressure of breathing gas constituents have profound effects on the health and ability to function underwater of the diver.
Aspects of physics with particular relevance to diving
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The main laws of physics that describe the influence of the underwater diving environment on the diver and diving equipment include:
Buoyancy
Archimedes' principle (Buoyancy) - Ignoring the minor effect of surface tension, an object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. Thus, when in water, the weight of the volume of water displaced as compared to the weight of the diver's body and the diver's equipment, determine whether the diver floats or sinks. Consequently the internal pressures in the body will vary with changes in posture, and with immersion depth in a denser fluid, such as water, following Pascal's law. In air, the hydrostatic pressure of body fluids is supported by the tensile strength and elasticity of the containing tissues. Fully immersed in water the hydrostatic pressure is almost completely balanced by the hydtostatic pressure of the water, and in partial immersion, the hydrostatic support imbalance is a function of the height of the emersed portion. Pressure needed to circulate blood through the vascular system is additional to static pressure.
The absolute pressure on an ambient pressure diver is the sum of the local atmospheric pressure and hydrostatic pressure.
Underwater vision
thumb|upright=2|Comparison of penetration of light of different wavelengths in the open ocean and coastal waters
Underwater vision is affected by the refractive index of water, which is similar to that of the cornea of the eye, and which is about 30% greater than air. Snell's law describes the angle of refraction relative to the angle of incidence.