Get trained
This page explains what training agencies, standards bodies and diving medicine bodies publish. It is not a substitute for training and certification from a recognised agency, and it does not replace medical advice.
Why physics is part of the course
Physics is written into the international standard for entry-level divers. Wikipedia (secondary) lists diving physics among the theory topics of ISO 24801-2, alongside equipment, medical and psychological problems and the diving environment. PADI's blog post on diving physics (Danielle Schofield, 5 June 2026) says Open Water Diver students learn how pressure affects the body underwater. This page only names and defines them. It has no calculations, tables or planning methods; those belong in a course.
Pressure rises with depth
SSI states that pressure increases by roughly one additional atmosphere for every 10 m (33 ft) of depth. PADI's post gives the same relationship as 1 ATA for every 10 m (33 ft) of descent in sea water.
The gas laws, as PADI states them
| Name | What it says, in PADI's terms | How PADI connects it to diving |
|---|---|---|
| Boyle's law | For a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional: as one goes up, the other goes down. | Presented with Archimedes' principle as the basis of buoyancy |
| Dalton's law | The total pressure of a mixture of gases equals the sum of the partial pressures of each gas in it. | Linked by PADI to the effects some breathing gases can have on deeper dives |
| Henry's law | The amount of gas that dissolves in a liquid is proportional to the pressure outside the liquid. | Linked by PADI to the nitrogen that dissolves in the body during a dive |
| Charles' law | Relates the temperature of a gas to its pressure. | Illustrated by a cylinder cooling in the water |
| Joule-Thomson effect | A gas cools as its pressure drops. | Gas cooling as its pressure drops through the regulator |
Training material from the US National Oceanic and Atmospheric Administration's diving programme groups Boyle's, Charles', Gay-Lussac's, Dalton's and Henry's laws together; we saw only the title of that slide set, not its contents.
Diving illnesses linked to pressure are described, from diving medicine sources, on decompression sickness and barotrauma. They need prompt medical care from a doctor.
Buoyancy: Archimedes' principle
PADI states Archimedes' principle this way: an object in water is pushed upwards by a force equal to the weight of the water it displaces. PADI presents Archimedes' principle and Boyle's law together as the foundation of buoyancy for divers. The equipment side of this is covered on BCDs and weights.
How water changes the senses
PADI's physics post also covers how water changes what a diver sees, hears and feels:
- Size and distance. Because of refraction, PADI says objects underwater look about 25 percent closer and 33 percent bigger than they are.
- Colour. Water absorbs colour with depth, and PADI says red goes first. Dive lights are covered on lights and signalling.
- Sound. PADI says sound travels about four times faster in water than in air.
- Heat. PADI says water draws heat from the body about 20 times faster than air does. Exposure suits are covered on wetsuits and dry suits.
Learning it properly
These definitions are a vocabulary list, not training. How they shape the way divers use their equipment and plan their time underwater is taught in a course by an instructor. For the terms themselves, see the glossary.