Kids: Why is vision different underwater?
Photo: Juin Hoo
When I’m swimming, I like to open my eyes underwater. I have great vision normally. So why does everything look hazy and indistinct underwater?
The reason has to do with the process by which you see. The outer surface of your eye – the cornea – collects light reflected from objects all around you. The cornea also focuses the light rays, causing an image of worldly objects to be formed on the retina of your eye. Then the retina sends the image of what you’re seeing to your brain.
When you’re under water, the cornea isn’t nearly as good as it is on land at bringing light to a focus. That’s because the focus is achieved by virtue of refraction – where the direction of light rays is changed as light passes from one medium to another. Normally, light passes from the air into the fluid in your eye.
But when you’re under water the light goes not from air to fluid but instead from one fluid to another. So refraction doesn’t take place nearly as effectively. Instead of coming to a focus and forming a clear image, the light collected by your eyes gives you the perception of an underwater world filled with indistinct shapes – shadowy – almost ghostly – at a distance that can be difficult to judge.
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