Why are objects blurry underwater?

Ever open your eyes underwater without goggles? You probably noticed that objects are really blurry, a fact that has nothing to do with the chlorine, though that will cause them to sting after a while.

It has to do with the fact that a ray of light traveling through a medium refracts, or bends upon entering another medium. However, its not merely because water refracts light that makes underwater objects blurry. In fact, the refraction of light in water is the basis upon which human vision is based.

This is because our eye is filled with a fluid mostly consisting of water. We are adapted to light rays traveling from the air into the fluid in our eyes, which is refracted at a specific angle. Our eyes therefore focus incoming light in a specific way, which is projected on the back of the retina, and fine-focused with the lens. So we are adapted to a specific incoming angle of light, due to this refraction.

Under water, light travels from water into another water based media in the eye. Therefore the angle of refraction is fairly small. The eye cannot focus the light so the image appears blurry (the focal point is in the wrong place).

So, how to fix this problem?

Well you already know that goggles work. But, how they work is explained by Snell’s Law, the index of refraction (n) multiplied by the sine of the incident (incoming) angle is equal to the index of the refraction of the second medium times the sine of the angle of refraction. A light ray traveling through air into water will bend the same amount, though the opposite way, as a ray of light going from water to air. In other words, light coming from the water into the goggles will refract at a certain angle. When this light enter the eye, it will refract back to almost the original angle it was traveling in the water. So, by placing a layer of air over the eye via goggles refracts light, received from from objects underwater, in a way they would normally look in the air.

(PS - special thanks to klaynos for reviewing this post)

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