Humans might be the dominant species on land, but we’ve fared less well underwater, partly due to our inability to see clearly. This meant a world of adventure was inaccessible to us until the invention of the humble swimming goggles. When this happened is unclear, but 16th century paintings show Venetian coral divers using rudimentary goggles made with wood and polished glass. We’ve come a long way since then: now goggles are designed by computers and made out of the most advanced materials. But why do we need them in the first place?
When light travels from one medium (e.g. air) to another (e.g. water) its path is bent, a phenomenon known as refraction. This bending is because light travels at different speeds in different materials. The refractive index of a material is a measure of how quickly the light moves through it, compared with the speed of light in a vacuum. As the density of the material increases, so too does its refractive index.
The refractive index of air, for example, is roughly 1, as there is very little that will slow it down. However, when light moves through water, which is much denser than air, it is slowed down to about 75% of its speed in a vacuum, and so water’s refractive index is about 1.33. The bigger the difference in refractive index between two substances, the more the light will bend as it moves from one to the other. This is why a straw in a glass of water looks like it has bent at the surface. It would appear to bend even more if you put it into a diamond (refractive index: 2.4).
To exploit this, our eyes have bulging, rounded corneas and a layer of liquid in front of the pupil. These bend and pre-focus the light, enhancing the eye’s optical power – its ability to focus – before the light reaches the lens. In humans, the cornea accounts for two thirds of the refracting (optical) power: the lens only accounts for the remaining third.
Our eyes are poorly adapted to seeing in water because the refractive indices of water and our cornea are so similar. This means the light is hardly bent at all before it reaches the lens. The lens is then the wrong shape to focus the light properly onto our retina – it is too flat. Instead, the image is focussed somewhere behind our retina. The effect is similar to a projector being too close to the screen.
We use goggles to correct for our lenses being too flat to see underwater. Goggles place a bubble of air in front of our cornea, allowing our eyes to bend and focus the light the right amount so that we can see clearly.
Humans have the ingenuity and innovative drive to create objects to help us overcome a debilitating problem underwater. Even if we cannot become the dominant species in the seas, we can still explore and observe to our heart’s content.
IMAGE: Ashfield swim school, www.timbauerphoto.com