How Do Scuba Masks Work?

Rok Valencic

There are a few things you must do right when picking your scuba mask and when using one to have a comfortable experience a good mask allows. But in order to do that, you first need to understand how do scuba masks work to make the most out of them.

Every scuba diving mask has two functions:

  1. Allowing you to see underwater
  2. Allowing you to equalize pressure inside the mask (and in your ears)

Let’s see how it works.

How a Scuba Mask Lets You See Underwater

As a diver and a physicist, my brain is already cracking knuckles and yelling refractive indexes and optical diagrams, but I decided to stop myself and keep it casual, hoping this will be a pleasant read 🙂

If you’ve ever opened your eyes underwater without a mask, you know that everything is blurry. That’s because our eyes are made to work in air, not water. And that is what a diving mask fixes.

In the simplest terms, the mask puts air back around your eyes, which allows them to regain focus and see. But let’s go into more detail and see how exactly that works.

Light Refraction Underwater

When you dive beneath the surface, light rays bend in ways that distort your vision. This is called light refraction.

Refraction is the bending of light as it passes from one medium to another. Underwater, light travels through water, which has a different density than air. This change in density causes light rays to bend at different angles, making objects appear larger and closer than they actually are and, often, blurry.

To illustrate this, you see it every time you look at something that’s submerged in water, like a straw in a glass:

Light refraction in a glass of water

However, your point of view in this image is still outside the glass, where there is air.

The light that bounces off the straw first travels through water, which distorts it a bit. Then, the air outside the glass ‘sharpens’ the light again before it reaches your eyes. That’s why you see the straw clearly but enlarged and shifted a bit.

If you were to submerge this glass in a larger body of water that you’re floating in as well, with your head underwater, this is what you would see:

Light refraction in water making our vision blurry

In this case, there is no air between the straw and your eyes that would correct the light distortion caused by water.

Geek Fact
Hypothetically, your eyes could focus the light refracted by water enough to see a sharp image without any air, but your corneas simply aren’t strong enough to do that. They would have to be able to increase their power by about 700% of what is currently considered their maximum!

A Diving Mask Refracts the Light Back

By putting on a diving mask, you add a layer of air between your eyes and the surrounding water. The air bends the light rays back to their original path, allowing your eyes to focus again:

Light refraction in water corrected with a mask

But in order for this to work, the air in front of your eyes must stay there. Which brings me to the second function of a scuba diving mask:

Why a Diving Mask Needs a Watertight Seal to Allow Pressure Equalization

A mask is useless without a watertight seal around a diver’s face. The silicon skirt keeps the air pocket around your eyes and nose and prevents water from entering.

In other words, it must withstand water pressure as you dive deeper without leaking.

The One-Way Seal

If you have ever tried exhaling through the nose harder while wearing the mask, you know the air had no trouble escaping.

On the other hand, if you tried inhaling, the mask stuck harder to your face.

This is due to the shape of the silicon mask skirt. And here lies the first part of this vital function.

A one-way seal on a scuba mask

Imagine if the seal held both ways. You could exhale a bit, then stop because the pressure in the mask would rise high enough to prevent further exhaling. Like when you’re close to fully inflating a pool lounge with your mouth.

That would mean that as you ascended from a few feet underwater, the pressure inside the mask would decrease, and the air inside the mask would expand, but it couldn’t escape. So, you would have to inhale it to equalize the pressure. We can safely agree that would be stupid and annoying.

Instead, the dive mask seal is designed to allow airflow only one way. As you descend, the pressure inside the mask increases, and you must exhale air through your nose to equalize.

This is the only reason why the nose must also be covered by the mask. There is no way to avoid this. If the mask wasn’t designed this way, the water would break into your mask as you started descending due to the rising pressure.

FYI
This is also the reason why you can’t dive using swimming goggles. The goggles would be pressed very hard into your eyes, or your eyes would be sucked out of their sockets as the pressure increases. This is called a ‘mask squeeze’. You do not want to try it.

Equalizing Air Pockets Inside Your Head

Why the nose must be enclosed in the silicon part of the mask and not the glass part (which would give you a better field of view) is because you must be able to get a hold of your nose to equalize your middle ear and other air pockets inside your head.

Even if you know how to equalize without pinching your nose, you still want this option just in case the hands-free technique fails.

What Should Keep the Dive Mask On

Understanding how the mask seal functions in relation to pressure difference and air exchange is important to understanding how to pick a mask that’s a perfect fit for you.

Some people will tell you that the water pressure keeps the mask on, not the strap. Others will tell you it’s the strap, not the pressure. The truth is, it’s both.

If it does, you get constant pressure from the outside, making equalization inside the mask unnecessarily harder.

Sometimes, you also need to lift the mask underwater to clear it of any water. You want to do that easily and not fight the strap.

If you exhale hard through your nose underwater, the seal will come loose somewhere to let the excess air out. When that happens, you don’t want the mask to shift on your face or come entirely loose. At that point, it’s the strap that’s holding the mask on.

But, because every strap can be adjusted to fit comfortably, the only important factor to consider when getting a new mask (or borrowing one), is how well the watertight seal holds without using the strap.

That’s why you always try on new diving masks without putting the strap on. You just gently press it over your eyes or put it on your face and inhale gently. That should make it stick comfortably to your face, indicating a good seal against it.

A scuba mask fitting without the strap

What Is a Purge Valve?

I decided to briefly include this as well, although it isn’t common on diving masks, and for good reason.

A purge valve is a silicon flap at the bottom of the part that covers your nose. Its idea is to allow you to let any water out just by breathing out through your nose.

Dive mask with a purge valve

Valves aren’t common on masks today because they aren’t reliable, fail often, make pinching your nose more difficult, and don’t add much to a mask’s functionality.

Learning to clear water from a mask is one of the basic tasks scuba divers learn in their course, and reliability is always more important underwater.

Which Mask Lenses Are the Best?

Every quality mask is made of tempered glass. Compared to standard glass, tempered glass is much tougher, is more scratch-resistant, and doesn’t break as easily. Even if it were to break, it would shatter into small, blunt pieces rather than sharp shards.

However, there are three lens types, which all affect your field of view and mask buoyancy.

You won’t feel much difference if you’re using a mask to snorkel twice a year. It can, however, make a difference if you’re regularly scuba diving, so picking the right one for you and the type of dives you plan on doing is important.

Single Lens Mask

A single lens scuba mask

Advantages

  • Single-lens masks offer a wide, uninterrupted field of view. They typically provide better peripheral vision since there is no bridge over the nose to obstruct sight.

Disadvantages

  • The large lens can sometimes result in a higher internal volume, creating a higher buoyancy and making it harder to clear if it floods.

Why Mask Buoyancy Can Be a Problem
If a mask has a lot of space inside, that means there is more air and, as a result, a higher buoyancy. This will always be pushing the mask up a bit.

If you ever feel discomfort below your nose when diving, it’s likely due to the mask’s pressure there as a result of buoyancy. In such a case, think about getting a mask with less internal volume.

Double Lens Mask (Twin Lens Mask)

A double lens scuba mask

Advantages

  • Twin lens masks have two separate lenses, which can result in a lower internal volume. This makes them easier to clear and often provides a snugger fit, enhancing comfort and reducing leakage.

Disadvantages

  • The nose bridge can slightly obstruct the central vision, but modern designs minimize this effect. This is me in the image above. I have never been able to see the nose bridge on my mask.

Tripple Lens Mask

A triple lens scuba mask

Advantages

  • Triple-lens masks have side panels in addition to the main front lenses, significantly improving peripheral vision. This gives a more panoramic view.

Disadvantages

  • The additional lenses can increase the complexity of the mask, making it more prone to leakage and harder to clear. They also add to the mask’s weight and internal volume.

Conclusion

While every mask will let you see underwater, only quality masks will provide a good and comfortable seal.

The best choice for a scuba diving mask, in my opinion, is a quality seal and low internal volume to reduce mask buoyancy.

Now that you know how scuba masks work, I hope you’ll be able to feel some more comfort and joy the next time you put on this unexpendable piece of diving equipment.

Rok Valencic

Rok Vale

I’m a sports enthusiast who enjoys spending as much time underwater as possible. Be that diving, snorkeling, swimming, or just falling off the surfboard. I’m a licensed PADI diver and a licensed fitness instructor. I also have a degree in physics to unnecessarily complicate my life.