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  • Tracy Gunn

Understanding about Water, Heat, Light, Sound and Gases in Physics IDC Dive Theory

Updated: Jan 11

In this final part of our IDC physics series, we will discuss water, heat, light, sound and gases and how they affect you while diving.

Water, the miracle compound. There are so many unusual traits that make this, literally, the most important thing on the planet. Where two highly reactive elements form to create something magical - H2O.


  • Water is polar

We all know the beautiful shape of water - H2O. It is the shape that determines water, with two hydrogen atoms on one side and an oxygen atom on the other, as seen above. Overall, water is neutral, but one end (the hydrogen end) is slightly positive, and the other (oxygen) end is slightly negative. This makes water a polar molecule, and it is because of this bond that water has unique properties.

  • Water is an excellent solvent.

Luckily for us because our lives depend upon it. Water´s ability to dissolve many polar and ionic substances makes it central to life. Nearly every form of life relies on water to transport nutrients and gases to its cells and remove waste. It is the primary component of blood and is crucial for many processes, from energy use to reproduction.

Again, luckily for us, many substances do NOT dissolve in water. If they did, there would be no life because we could not form cellular structures.

Non-polar molecules like oil and fat lack partial positive or negative charges and, as such, are not attracted to water molecules. This is why oil separates in water. Soap compounds bond with both polar and non-polar molecules, and it is this link that allows water to wash dirt away.

  • Water has a high heat capacity.

Waters polar molecules allow it to have one of the highest heat capacities of all naturally occurring substances. Heat capacity is the measure of the amount of heat that must be added or subtracted to make it change temperature by a certain amount. Air has low heat capacity, and it is for this reason that a comfortable air temperature will quickly become uncomfortable in water. Air does not carry heat away from your body as quickly as water can. To raise a volume of water a specific temperature, it will take about 3200 times as much heat as it would for the same volume of air.

Because it takes a lot of energy to raise the water temperature a degree, water can help regulate the temperature in an environment. A pond, for example, will be able to have a relatively constant temperature day to night, regardless of changing atmospheric temperature.

On a grander scale, water is crucial in regulating the world's climates. Major currents help moderate temperature by carrying heat from warmer equatorial regions towards cooler polar ones.

  • Water has a high heat of vaporisation.

A significant amount of heat is required to break the polar bonds of water before heat can increase the temperature of water. Because of these polar bonds, water evaporates more slowly than any other common liquid. This is referred to as having a high heat of vaporisation because sufficient heat must be used to break the polar bonds. Humans and other animals use this phenomenon to cool off. Water is converted to steam when the heat of vaporisation is reached. The water in the sweat absorbs excess body heat and is released into the atmosphere. This is known as evaporative cooling.

Water also has a latent heat of fusion, meaning when water freezes, it releases high quantities of heat, and when the ice melts, it absorbs large quantities of heat.

  • Water has cohesive and adhesive properties.

These polar bonds mentioned above make the water surface slightly cohesive, so it resists separation and penetration. It is this surface tension that holds water droplets together and is strong enough to "float" certain insects or objects much denser than water. Unfortunately, pollutants can be known to break down surface tension.

Water also has adhesive properties. This property allows it to stick to substances other than itself. These properties allow for fluid transport in many life forms. Nutrients can be transported against the force of gravity to the top of a tree.

Cohesion is an attraction to like molecules - water to water

Adhesion is an attraction to unlike molecules - water to glass

  • Water is less dense as a solid than as a liquid.

A property that is far from typical is the density of water when it freezes. Most liquid substances get denser when they cool and eventually turn into solids. These solids are even denser and sink, accumulating on the bottom, and the liquid becomes solid from the bottom up.

Fortunately, water does not do this. As it cools, it becomes denser, but when the temperature drops to 4ºC/39ºF, water begins to solidify into ice. As it freezes, the polar molecules force the molecules into a crystalline structure that spaces them farther apart than when the water is in its liquid state. This means that water is less dense in its solid form (ice) than in its liquid form, which is why ice floats.

Why is this great? Because it is a major influence on the world´s climate. Floating ice insulates and retains heat in the water underneath. This slows the freezing process. It keeps ponds, lakes, and oceans from freezing solid so that life can continue to thrive underneath. If ice sank instead of floating, oceans could be frozen solid, and most of the Earth's water could be frozen.

  • Density stratification

Until its freezing point, liquid water behaves like a typical liquid, meaning it becomes denser as it cools. The water forms into layers of differing densities, becoming colder as depth increases. This formation of water into layers is referred to as density stratification. As you descend from one layer to another, you may experience abrupt drops in temperature. This is called a Thermocline; in still waters, these temperature changes may be close together.

Dissolved salts may also increase water density and cause density stratification. With enough dissolved substances, warmer waters may be denser than cooler waters and lie deeper, as experienced in the cenotes here in Mexico. A diver may descend and experience an abrupt transition from the cooler fresh water into warm salt water below. This is called a Halocline.


Besides its polar characteristics, water absorbs more heat because it is denser.

Heat transfer occurs via three means.

Conduction - refers to heat transmission via direct contact.

We understand that when a substance is heated, the molecules move faster. Imagine a handle on a saucepan. The excited molecules transfer some energy up the handle via direct contact. If the handle is a good conductor (a substance that efficiently transmits heat by direct conduct), it will heat quickly until the entire handle is heated to a uniform temperature.

Air is a good insulator (substances that do not conduct heat easily), which is why dry suits are better protection from the cold than wetsuits (and using Argon gas makes it even more so).

Water is an excellent conductor (substances that transfer heat better than others). It conducts heat away from the body 20 times faster than air due to the water molecules being closer together. This is why you will need insulation while diving in all but the warmest water.

Conduction affects us the most while diving.

Convection - involves heat transmission via fluids as particles circulate in currents.

A fluid that flows is either a gas or a liquid but not solid. As fluid is heated, it becomes less dense and tends to rise. In heat transfer by convection, the particles in a liquid or gas speed up as they are heated. This causes the particles to move apart, and the substance becomes lighter. As the heated substance rises, the cooler, heavier substance moves down. These currents exchange heat through this movement, causing a continuous flow that draws heat away faster than pure conduction.

This is important in diving because as your body heats the water, the water will become less dense and rise with cooler water replacing it, even when there is no current or physical movement. As we do not have enough body warmth to warm the entire ocean, we wear wetsuits (or dry suits). These trap the water or air between the suit and the body and reduce the effect of convection while our body warms the air or water trapped within via conduction.

Radiation - refers to heat transmission through space via electromagnetic waves.

This type of heat transfer affects us the least while diving. Radiation heat is what we feel from a fireplace or the sun. Some high-powered strobe lights on cameras can emit radiated heat.


Light is a form of electromagnetic energy that travels in waves; the length of the wave is determined by its energy, which determines how we classify the type of electromagnetic energy.

The sense of sight is the perception of this electromagnetic energy (light). Humans can only perceive a very narrow range of wavelengths ranging from 380 nanometres to 800 nanometres. Differences within this range are perceived as colours.

Colours on the red end of the scale have less energy than colours on the blue end of the scale. Infrared is very low energy. and ultraviolet is very high energy.

Some wavelengths are not visible to the human eye, such as ultraviolet light, x-ray, etc.

Light emitted or reflected by an object is collected by the eyes, allowing us to "see". Objects absorb some wavelengths and reflect others. What we see as colour is based on the reflected light wavelengths. These waves of energy collected by the eyes are turned into electrical impulses and sent to the brain via the optic nerve.

Refraction - is a change in direction when light passes from a medium of one density to a medium of a differing density - like from air to water or vice versa.

Underwater, the eyes continue to function, but the light itself changes. Water slows the speed at which light travels. This change in speed causes the light to bend (or refract). If you look at a straw in a glass, it will appear to "bend". The denser the medium the light is travelling through, the slower the speed.

When diving with a face mask, close vision is affected by refraction. Distortions affect the diver´s hand-eye coordination and the ability to grasp objects. The light that comes to a diver´s eyes underwater is refracted through different mediums - water, glass, and air.

Generally, objects appear 25 % closer than normal and 33 % larger (or 4:3 magnification) because of refraction. When wearing a diving mask, the light travels through the water, the mask window, and the air inside the mask. Each has a different density, and light will refract through these interfaces. These distortions caused by the mask vary largely with the distance viewed. Closer than 1.2 meters, objects appear closer than they are. More than 1.2 meters, overestimation occurs—this degree of error increases in turbid water.

Interestingly, light does not refract when it enters a medium of differing density if it enters at a perpendicular angle.

This is why some cameras use dome ports, as this eliminates refractions because all light enters at a perpendicular angle.

Turbidity - is the relative concentration of suspended particles

This can be caused by plankton or stirred-up sediment (silt), rainwater runoff or pollution. The higher the turbidity, the less light penetration occurs and the less visibility for the diver.

Diffusion - is the phenomenon where water scatters and deflects light.

Because light scatters off particles when it penetrates the water, the light is spread more evenly, so shadows are either reduced or eliminated. For this same reason, things far away can appear indistinct or fuzzy. The higher the turbidity, the more diffusion.

Visual Reversal - Although refraction makes things look closer than they are underwater, turbidity can make them look farther away than they are, and an overestimation of distance can occur.

The perception of distance underwater is very likely to be inaccurate.

A rule of thumb is

  • The closer the object, the more likely it will appear to be closer than it is.

  • The more turbid the water, the more likely it will appear farther than it is.

Reflection - when light waves strike something and bounce back.

Without reflection, we would not see anything. It is only by reflection that we, and most objects in the physical world, can be seen, and this is because we make our presence known by reflecting light into the eyes of those looking our way. (4)

The angle at which light strikes a mirror is called the angle of incidence. It bounces off at the same angle. Some substances, like water, may reflect or transmit light depending on the light's angle. As the angle decreases, more light reflects proportionally, and less penetrates the water. Because of this, the best natural light, especially with respect to photography, is generally found between 10 a.m. to 2 p.m. when the light is directly overhead at a steep, nearly perpendicular angle, and most of the light penetrates.

Colour Absorption

Water transforms light passing through it into heat. This does not happen evenly, as wavelengths with less energy are absorbed first. This means that colours on the red end of the spectrum are the first absorbed and, as such, tend to disappear rapidly as you descend. Beneath 4 meters, you will not see much red. After red, the water more readily absorbs orange, yellow and then green. In very deep water, only blue and violet are visible.

Colour is not absorbed only by depth but by the total distance light travels through the water to your eye. So, being 5 meters away from an object sitting at 5 meters means that light travels 10 meters to reach your eye.

Fluorescents retain colour underwater because the wavelengths are not common, and they emit colour when stimulated by light of any shorter wavelength.


Light and sound both travel in waves.

The difference is that:

  • Light is electromagnetic energy that can exist apart from matter and pass through a vacuum.

  • Sound is mechanical energy that can only exist and travel through matter.

Sound results when something sets in motion a wave or pattern of waves in matter. This wave or pattern may be transmitted from one medium into another. When the wave reaches your eardrum, transmitted by water or air, your ear converts some of the energy into nerve impulses that your brain interprets as sound.

Sound travels best through solids or liquids because they are denser. Denser materials have more tightly packed molecules. However, it is not the density that transmits the sound but rather the elasticity of a substance. Generally, denser materials have more elasticity, but this is not technically accurate. Think of how a blanket muffles sound rather than transmits it.

  • Air, at 0ºC, sound travels around 332 meters per second

  • In freshwater, at 15ºC, sound travels at 1410 meters per second

  • In saltwater, at 15ºC, sound travels at 1550 meters per second

As you can see, the speed of air is greatly influenced by the medium through which it travels, as well as temperature and pressure.

The speed of sound underwater is about four times faster than in the air. This can often make it difficult to determine where sound is coming from. On land, sound reaches your ears with a slight difference in intensity and time. Underwater, intensity and time appear the same, and sound is often perceived as coming from above.

Differing densities resist transmission of sound, such as from air into water or vice versa. Sound loses a lot of its energy, transferring between the two. Because of this, you cannot hear someone yelling at you from the side of a pool when you are just under the surface.

We talked above about density stratification, where temperature and dissolved salts form layers in water. Sound travelling between these layers will also be resisted due to the differing densities. A thermocline or halocline can greatly affect sound transmission.


In their natural state, many elements exist as gases, and because gases mix easily, they usually occur in nature mixed rather than in isolation. The best example of this is the air that we breathe. Air is a mixture of 78% nitrogen, 21% oxygen and about 1% other trace gases; to simplify calculations, this 1% is usually treated as nitrogen. The reality is air consists of nitrogen, oxygen, argon, carbon dioxide, neon, helium, methane, krypton, xenon, radon, and carbon monoxide. We will look at the individual characteristics of these gases and how they relate to diving.


The biggest constituent of air is, by far, nitrogen, being more than all the other gases put together.

As an element, it is very common and readily reacts with other substances. As a result, it is found in many natural compounds, such as proteins.

As a gas, two atoms join together (diatomic) to become a single molecule (N2) that is inert (does not react with other substances) during diving. However, plants use N2 in biochemical processes, so it is not inert (in plants).

We are not plants; our bodies do not use nitrogen in respiration.

Our bodies use oxygen to convert food to energy (metabolism) and produce carbon dioxide as a waste product.

Because we do not metabolise nitrogen, what goes in, must come out. Several problems can occur during diving as a result of breathing nitrogen under pressure. Emphasised by Daltons Law and Henrys Law

It occurs when the partial pressure of nitrogen (PN2) is 2.37 - 3.16 bar (20 to 30 + m) and above of absolute breathing air and is caused by the partial pressure of nitrogen in the blood.

When we breathe nitrogen under pressure, it interferes with the transmission of signals to the central nervous system, causing an intoxicating effect known as "gas narcosis", "the martini effect", or "raptures of the deep."

It is a disorder in which nitrogen, dissolved in the blood and tissue by high pressure, forms bubbles as pressure decreases.

When we breathe nitrogen under pressure, it dissolves into the body tissues. The greater the pressure, the denser the gas, the more nitrogen is dissolved into the body. This nitrogen builds up until, if you remain at a depth long enough, it reaches a state of equilibrium (saturation) with the air that you are breathing.

Problems do not occur because of the descent but rather upon the ascent. As long you follow your dive tables or computer and ascend at a safe rate, you will minimise the risk of decompression sickness caused by excessive supersaturation.

More on this in IDC physiology.


Oxygen is a very abundant element essential for the body´s metabolic process that sustains life. While the body can store food for later use, it cannot do so with oxygen. If the body is deprived of oxygen, permanent damage and death can occur. Also, breathing oxygen under high pressure can cause oxygen to become toxic.

Because of the body´s metabolism of oxygen, it is not generally a concern with respect to decompression sickness. However, as we discussed in Dalton's Law, oxygen toxicity occurs when the partial pressure of alveolar O2 (PAO2) exceeds that breathed under normal conditions. This acceptable limit is 1.4 bar/ata (1.6 bar/ata at rest in technical diving), which you will not reach diving recreational limits on air (21%), but it is possible diving with enriched air or technical diving.

Oxygen can also contribute to gas narcosis, so we treat air and enriched air as equally narcotic.

Hypoxia (insufficient oxygen) is a state where insufficient oxygen is available at the tissue level to sustain adequate homeostasis.

Hyperoxia (excessive partial pressure) is a state of excess supply of O2 in tissues and organs.


Argon, like helium, and neon, exists as an individual atom. It is a physiologically inert gas that does not bond with itself.

It is a very dense atom that is an excellent insulator due to its large molecules and low specific heat. For this reason, recreational, technical, and commercial diving uses it in dry suits for insulation that is superior to using air. Argon is more resistant to compression as the diver goes deeper, further assisting the effectiveness of the existing thermal undergarment.

Argon is a gas that is too soluble in fatty tissue. It would produce too many decompression bubbles that could potentially be lethal. This is why it is not used as a breathing gas.

Argon is also less affected by depth changes than regular air. Less venting or adding argon is needed to maintain neutral buoyancy when the diver ascends or descends.



Carbon dioxide

is an odourless, colourless, and tasteless gas, although, under high concentrations, it can take on an acid odour and taste.

Besides being a greenhouse gas that is a by-product of pollution, it is also the main waste product from respiration.

  • Hypocapnia - insufficient carbon dioxide - can cause unconsciousness without warning

  • Hypercapnia - excess carbon dioxide can cause difficulties in breathing and air starvation at depth.

Carbon dioxide levels are important as they control the breathing in humans.

Poisoning can occur by

  • Overexertion

  • Deep diving

  • Inadequate respiratory effort due to breathing resistance (tight wetsuit, malfunctioning reg) (Hypoventilation)

  • Air supply contaminated by exhaled gases (CO2 scrubber failure in rebreather)

Carbon monoxide

It is a gas that is found in nature only rarely. Mainly, it is artificial and a by-product of fossil fuels. Although it has no taste or smell, it generally occurs with other compounds that do. Carbon monoxide rarely gets into breathing air, but it can happen if the intake valve is placed too close to the engine exhaust or if the lubricating oil in a malfunctioning compressor becomes hot enough to partially combust, causing CO.

This is why you do not dive with a tank that has a foul taste or smell.

Trace amounts of CO can be toxic at depth due to partial pressures (Dalton Law)


Helium is an inert gas that is relatively rare. The element is so stable that it does not form a two-atom gas molecule with itself. It is never found in compounds. Therefore, the gas exists of individual atoms and not molecules.

Helium is less narcotic than nitrogen at the equivalent pressure, and the low density makes it easier to breathe at depth, making it more suitable for deeper dives than nitrogen. This is important in technical and commercial diving. Various blends such as Heliox (helium and oxygen) are used for commercial divers, and trimix (oxygen, helium, and nitrogen) are used in technical diving deeper than 50 mt.

There are various concerns with helium, however.

  • It is equally able to cause decompression sickness. Being a light gas, it diffuses more quickly than oxygen or nitrogen, dissolving into and out of your tissues faster. Therefore, the decompression time required when using helium will be longer than an air or enriched air dive.

  • Helium conducts heat away faster than oxygen or nitrogen. Not an issue for breathing but ineffective for dry suit inflation

  • For commercial divers, helium transmits sound faster than air, causing speech to be distorted—an issue when using electronic voice communication.

  • For commercial divers. High-pressure nervous syndrome (HPNS) affects the central nervous system. The narcotic effect of nitrogen can be used to counteract this.


The lightest and possibly most abundant element in the universe is, unfortunately, at this moment, not ideal for diving. Although research was conducted into the possibility, it was found that the gaseous form of hydrogen is highly reactive and reacts violently with oxygen to form water. Great care would have to be taken during gas switches to avoid a reaction inside the lungs.

Hydrox (a mix of hydrogen and oxygen) and hydreliox (hydrogen, oxygen, and helium) have been used to develop dive procedures to depths of 500 mt and 700 mt.

The first uses of this gas are attributed to Arne Zetterstrom, who showed that hydrogen was perfectly usable at depth. He, unfortunately, died during a demonstration dive due to a fault in the surface equipment.

Hydrox may be used to counter the effects of HPNS that commonly occur during deep dives, and a simulated dive in a decompression chamber took Theo Mavrostrom down to 701 meters on November 20, 1990

Real-life applications of hydrox allow for no more than a 4% mix of oxygen due to the reactive nature of the two, and in July 2012, in memory of Arne Zetterstrom, divers used this hydrox mixture (H2 96% and O2 4%) to complete a dive down to 40mt. The limit they were able to dive using the oxygen-lean mixture.

Currently, the only applications in diving are experimental and only applicable to commercial diving, with no foreseeable application to recreational or tech diving in the near future.


Neon, like helium, is an inert gas that does not bond with itself, is never found in a compound, and the neon gas contains individual neon atoms, just like helium. Also, like helium, it is less narcotic than nitrogen. Neon, however, does not distort the diver's voice, has superior thermal insulating properties, and because it is slightly denser than helium, it has decompression advantages. This slight density difference means neon is not as easy to breathe at depth as helium but is still light enough to dive up to 155 mt.

So why are we not using it?

Neon will likely become a common breathing gas for commercial or tech diving over the following decades. While the application in diving, for now, is still experimental, this will very likely change.

Until now, it has been expensive to produce pure neon, but with helium supplies predicted to run out by the end of the century, helioneon is an excellent alternative.

Helium supplies are produced mainly in the Southwest United States and must be shipped everywhere needed. Neon (or helioneon) is a by-product of the distillation of liquid air, and this is manufactured worldwide to prepare liquid oxygen for industrial processes and liquid nitrogen for refrigeration (10). What is actually obtained is a mixture of neon and helium, or crude neon. A relatively easy and inexpensive process that is set to replace helium in diving as helium becomes scarcer and more expensive.

While neon is suitable for deep diving, it has no advantages or properties for recreational diving.

Now you are ready for some testing on Water, Heat, Light, Sound, and Gases.

Try the exam below.


Here are links to all the physics blogs

And to all the exams

Water, Heat, Light, Sound, and Gases Exam

Archimedes' Exam part 1

Archimedes' Exam Part 2

Under Pressure Exam

Boyle's Law Exam Part 1

Single-Level Depth Changes

Boyle's Law Exam Part 2

Multi-Level Depth Changes

Charles' Law Exam

Henry's Law Exam

Dalton's Law Exam

(1) NOAA Diving Manual. (1977). Physics of Diving.

(2) Utah Education Network. (2002). Literacy.

(4) Physics Tutorial: The Role of Light to Sight. (2021). The Physics Classroom.

(5) The Encyclopedia of Recreational Diving (3rd ed.). (2008). PADI.

(6) Divemaster Course Instructor Guide (1999 edition). (2005). PADI

(7) 8.1 Underwater Vision - What Visual Changes Occur During Diving? Is There Any Way to Avoid Them? | (2021). Grupo Portuguès de Ergoftalmologia.

(8) Argon Fills - Diver Dan's. (2021, January 31). Diver Dan’s.

(9) Moon, R. E. (2022, March 30). Gas Toxicity During Diving. MSD Manual Professional Edition.,or%20low%20air%2Duse%20rates.

(10) Deco. (2000, November 13). NEON as a dive gas - - -. ScubaBoard.


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