The Physics of Diving (Heat / Light / Vision / Sound)
In this section of my "Becoming a PADI Instructor" Blog Series, I am going to be focusing on the Physics of Diving. This section and the Physiology section is where I'll probably be spending the majority of my time as I was told by most of the instructors I've talked to that if I have those two sections down well the rest should be fairly easy. I'm mostly using the PADI Encyclopedia of Recreational Diving and IDC-Guide for my research. Otherwise I'm using Google to find academic resources to get a greater understanding of things I want to learn more about.Water & Heat
Have you ever been on a dive in 81 degree waters for a 60+ min dive in just a shorty or full 3mm? After about an hour or so even in warm waters you'll find yourself starting to get a little cold; at least I do. This is because water has one of the highest heat capacities. Which means it takes a lot of heat to added or removed to make a significant difference in temperatures. The water literally sucks the heat away from your body (Around 20 times faster than air!). It does this via 3 different methods.
- Conduction - heat transmission via direct contact (Affects dives the most)
- Convection - heat transmission via flowing liquid/air (When you're not wearing a wetsuit, your body will heat the water around you making it rise only to let cold water surround your body again absorbing even more heat)
- Radiation - heat transmission via electromagnetic waves (least affect underwater, some dive lights will produce enough heat to feel from a distance)
Water & Light
I'm sure as a diver or friend of a diver you've seen plenty of photos that just tend to be blue or green and not much else; yet when you watch Planet Earth on the Discovery channel every scene you see underwater at no matter the depth seems to be the most majestic thing you've ever laid eyes on. Its beauty often brings you to tears (okay, probably not that majestic).
The reason for these bland blue/green photos is because of waters Absorption of colors wavelengths. Brighter colors have lower wavelengths and less energy and are thus absorbed by water much quicker than darker colors.
The above chart shows the wavelengths for your typical colors, from top to bottom is the order in which you'll also lose those colors as you get deeper underwater. Below are the ranges in which these colors start to fade and disappear by.
| Starts to go at | Gone by |
RED | 1m | 5m |
ORANGE | 5m | 16m |
YELLOW | 22m | 30m |
GREEN | 50m | 76m |
VIOLET | 4.5m | 30m |
Table above from IDC-Guide
Only 20% of sunlight reaches 33ft / 10m. This is because of Diffusion, the scattering of light from the water.
Water and Light and Vision
Now that we've discussed colors, we can proceed on to how water and light affects your vision.
Refraction is when light passes from a medium of one density to a medium of a differing density (air to water, or water to air) . The speed of light is changed by the density of the medium and causes the light to change paths. The only time light doesn't shift its path is if the light passes at an angle perpendicular to it's surface. This is why underwater photographers use dome housings on their cameras.
When we are diving, refraction affects us mainly when light goes from the water in to the air space in our masks. I know you've tried to reach out and grab something in front of you and totally whiffed it! Don't lie; I know you did. That's okay though because it's not your fault. The good news is as you continue to dive your subconscious will start to correct it for you!
The refraction of light from water to our masks cause an object to appear 25% closer or 33% larger than it actually is. [Insert "The fish was thiiiiiiss big" story]. This is at a 4:3 ratio; meaning an object that is 4 ft away will only appear to be 3 ft away.
If you're a landlocked diver like myself you've probably got more experience in the next property of light and water than most ocean divers because of the not-so-clear waters. Turbidity is the concentration of suspended particles: silt, sand, algae, etc. Turbidity is the main factor in Visual Reversal .
Visual Reversal has the opposite effect of refraction, causing objects to look much further away than they are. The reason for this is our brains. Our brains base distance off of how diffused an object is and its level of contrast. Because air absorbs light much slower than water light has to travel much further before an object is diffused and has less contrast. Now put yourself in some water with a lot of turbidity combined with the absorption of light (color) and objects can appear very diffused and blurry. Your brain is trained to believe that object is much further away! Pretty cool, huh?
OoOO, so pretty! Where does the sky end!? What a beautiful reflection! Reflection is another property of water and light. Reflection is primarily based upon the color of the object the light hits. If the object is darker or black the object will reflect very little light. If it's white or a lighter shade more light will reflect. Light reflects even more so at shallower angles. This means that during the morning or evenings you'll have greater reflections off water than during the morning or evening. If you remember earlier when I was talking about using Dome Ports for cameras very little light is refracted when it hits the object perpendicularly and the same is true for reflection. Because of this you'll find the best time to shoot photos is between 10 a.m. and 2 p.m when less light is being reflected from the surface of the water and more light is making it to where you're taking your photo.
Water and Sound
Sound, like light, is a wave but unlike light it requires matter and cannot operate in a vacuum because it is a mechanical wave. Sound waves must collide with molecules in order to travel.
In the animation above, the circles heading outwards from the center are sound waves and the dots are any sort of molecules; like air. Typically the more dense those molecules are the better and faster sound waves travel through them. This is not always true because ultimately the elasticity of an object determines how well sound travels, but it just so happens that more dense objects have greater elasticity. Think back to your childhood when you would snap rubber bands and ropes to make sounds. Rope is more dense than the rubber band but the rubber band makes better sound because of it's greater elasticity. The same is true for water as it is more dense and has greater elasticity.
How does this affect our diving environment? In order to answer that question you must understand how our brain interprets the direction and intensity of sounds using our ears. When you're outside and listening to what is going on around you with your eyes closed it is easy to determine which direction a sound is coming from. This is because those sound waves are traveling much slower through air allowing the sound waves to hit each ear at a different time at a different intensity. Your brain receives those signals from the ears and then judges direction and loudness from those signals. If the sound is coming from your right side the sound waves will clearly hit your right ear much sooner than the left ear.
Sound waves travel 4 times faster underwater than through air so the waves hit each ear so much closer together that the brain has trouble interpreting which one it actually hit first causing it [the brain] to typically think the sound is coming from directly above. Also because sound waves travel so easily through water you can hear sounds coming from much further away than in air but will sound much closer than they actually are. Think back to listening to a boat going over the water when you're diving and looking around and not being able to see it anywhere near you.
Sound has trouble traveling from a material of one density to that of another. Imagine a thermocline (different temperatures) or halocline (Salinity gradient within a body of water ) or from the top of the water to underneath at a short distance. In any of these cases imagine sound like a bullet flying through the air freely and then smashing in to a wall or target of some sort. This causes the bullet to slow down due to resistance. The same is true for sound, which is why you can't hear the person screaming from the surface at you even though you're only a few feet below them.
If you have any questions or would like me to cover a certain topic or scenario with any of these properties please feel free to comment. In order to keep these posts organized and short(er) I will continue with some of the laws of physics in upcoming posts instead of cramming them in here.
On to Becoming a PADI Instructor: Bouyancy
Disclaimer: I am not a PADI Instructor and am writing this up as a learning aid for myself to better understand and explain the theories as I learn them. You should NOT be using these as your sole resource for learning how to dive or dive education. I suggest you sign up for a PADI or a respective company course to continue your diving education. If you find any errors in these posts, please feel free to contact me. Thank you.