One of the first things you notice as a new scuba diver is the surprising lack of color. Discovery Channel specials and magazine layouts have brilliant colors. Where’d they all go?

Don’t feel ripped off. The fact is, water absorbs light rapidly. So rapidly, that after only 300 ft (80 m), no visible light remains. This is far deeper than you’ll ever go, but this absorption is very important at all depths.

The visible light spectrum can be broken up into the familiar constituent colors (for example, see the too-familiar picture of Newton with a prism). From least to highest energy, red, orange, yellow, green, blue, and violet. This order is important, because it is the lowest energy colors that are absorbed first in water. This chart below shows the depths at which different colors are absorbed. These are approximate, as water clarity and turbidity affect color absorption.

What is doing the absorbing exactly? All water contains microscopic particles. Light strikes these particles and scatters, with some of the light absorbed. What remains is what color you see. As the light travels farther, only blue light remains, with it eventually being absorbed as well¹. Of course, flashlights reintroduce “white” light, which contains the entire spectrum, making all colors visible again.

To illustrate, I’ve recreated how some common items would look at different depths.

Notice how the red is removed from the orange, leaving a little color while the blue text is unaffected.

1. Different particles scatter light differently, so some water appears more green than blue.

Overpressure relief valves are pretty much standard on today’s BCDs. You can identify them as the vent-like areas, either on the shoulder and / or bottom rear of the jacket. Often they have a ball-and-string assembly attached called a “dump valve.” Pulling on this will release air from the BCD through the valve. This allows the diver to vent air easily from almost any orientation underwater, without awkwardly holding their inflator hose toward the surface.

As their name implies, the primary purpose of overpressure relief valves is to prevent the accidental over-inflation of BCDs. BCDs contain what are called bladders, or basically, air bags. These air bags have a finite amount of air they can hold, and exceeding this amount, either by over-filling it or by ascending to a depth that increases the air’s volume enough, would ordinarily cause it to rupture.

The trick is to put a hole in the bladder, but a hole that only leaks air when there’s too much of it. This is accomplished by holding a plug in place over the hole with a spring. This plug effectively corks up the hole. The spring has to be perfectly strong enough to hold the plug in place while air enters the BCD, but weak enough so that the plug pushes out, releasing air, when the pressure inside the bladder gets too high.

BCDs have some maximum pressure they can withstand. This is easy enough for the manufacturer to find out. Just fill the BCD until it explodes. The pressure right before it pops is the maximum pressure, although they’ll lower this a bit for a built-in tolerance.

With this maximum pressure p_max, we can figure out the strength, or stiffness the spring should be. We do this using Hooke’s law, which states that the force of the spring is equal to the deformation of the spring times a spring constant (the stiffness of the spring), or,

F = -k x,

where k is the spring constant that we want to find. We know what force is keeping the plug in place, it is the maximum pressure times the area of the part of the plug that feels the BCD’s air pressure (pressure is force per unit area), or p_maxA. The spring is also be deformed slightly to hold the plug firmly in place. How much the spring is deformed gives us x, which tells us the force exerted by the spring on the plug. With that number we can rearrange and compute the spring constant as

k = -p_maxA / x.¹

If the pressure increases, the spring won’t be strong enough to hold the plug in, and the BCD will release air. You can also manually override the valve by pulling the plug yourself, which is what you are doing when you pull the cord.

Like most parts on a BCD, the overpressure valve is an incredibly simple device. The simplicity of this device helps make BCD inspections a fairly easy process, especially when compared to overhauling a regulator.

1. We are ignoring the mass of the spring and the plug.

It can take a while to get used to hearing sound underwater. You are constantly receiving visual input through an entire dive, so your brain learns to compensate for refraction and other visual properties of water. However, audible input is not constant, so each time a boat drives by it catches you by surprise. What makes sound different underwater?

The largest difference is speed. At the surface, sound travels at about 340 m/s. In sea water that increases to about 1500 m/s, over four times faster! You constantly use the speed of sound to distinguish the audible world around you; your brain is unable to cope with this drastic increase, and thus noises sound like they are coming from all over.

With the relatively slow speed of sound in air, your brain uses the distance between your ears to pinpoint the sound’s origin (through a process called triangulation). For example, let’s say you hear a sound immediately to your left. The sound will reach your left ear before your right. If your ears are a distance of 8 inches = 0.2032 meters apart, we can determine the time difference as t = 0.2032 m / (340 m/s) = 0.000597 s. Small, yes, but large enough for your brain to detect. If the sound reaches your ears at the same time, then the sound is coming from directly in front, above, or behind you (ignoring sound waves bouncing off walls). Visual input fills in the missing blanks to pinpoint the sound’s origin.

Now let’s repeat the same calculation underwater. The time differential from a sound immediately to your left is t = 0.2032 m / (1500 m / s) = 0.000135 s; below the threshold your brain can distinguish. Combine this with no visual input directly in front of you, and your brain just assumes that all sound is coming from directly on top of you. Humans don’t like loud noises (such as those from boat engines) from directly above, so this tends to trigger fear reflexes.

Sound bounces off walls and objects, further aiding your brain in the triangulation process. The ocean is a much wider expanse in which sound travels freely, further altering your underwater perception.

This time we discussed how the location of sound underwater is perceived. Next time we’ll look at how the actual sound is altered.