Why Refraction Matters

Refraction is essential to eyesight. Without this process of bending waves of light, your world would be a blur. You couldn’t read a bestseller or admire stunning scenery.  

Eye doctors are experts at diagnosing foul-ups in refraction. If they find one, they call it a refractive error. 

But what exactly is refraction? How does it work and why is it so important to your vision? That takes a bit of explaining. 

Refraction is the bending of a wave as it passes at an angle through something with a different density. Refraction can happen to any energy that travels in waves, like sound, heat or light. We’ll zero in on refraction of light in this article because it’s so important to vision. 

Refraction has three requirements: 

1. A transparent medium
2. Different densities
3. Waves moving at an angle

Let’s talk about each of those separately.

A transparent medium

Transparent means see-through. Medium means a substance that waves pass through. Thus, a transparent medium is a see-through substance like air, glass, plastic, water — or the cornea and lens in your eye. 

Different densities 

Think about a ray of light traveling from the sun to the earth. If this ray hits the moon, it bounces off, creating the reflection you see in the night sky. The moon is solid rock, which is so dense that light can’t pass through it. 

Something else happens if the light hits something with a lot less density. 

Imagine our ray of light missing the moon. It keeps moving in a straight line at 186,000 miles per second until it reaches the earth’s atmosphere, which is transparent. Here, the light ray slows down a bit because our planet’s air is more dense than the vacuum of space. 

The same process happens when light passes through the lens of a camera. A lens is far more dense than air. Any light waves passing through the lens will move slower, causing the waves to refract (the scientific word for “bend”).           

Waves moving at an angle

If our single ray of sunlight hits the surface of the Atlantic ocean straight down at an exact 90-degree angle, it will pass through the water — but it will not bend. This is because refraction of light happens only when the rays hit the transparent medium at an angle. 

This angle is crucial to the lenses used in microscopes, telescopes and eyeglasses that correct vision. Convex and concave lenses use curved surfaces to refract light in specific directions to reveal tiny life forms and the distant reaches of the cosmos. And they give us better eyesight. 

READ MORE: Infographic: How eyeglasses work and a history of glasses

Light passing through water creates the most common, everyday examples of refraction. If you fill a glass with water and place a spoon in it, the spoon will appear to bend at the surface of the water. This refraction happens because light travels just a tiny bit slower through the water than it does through the air around it. 

The same concept makes it extremely difficult to catch a fish with your bare hands. Because of refraction, your eyes don’t see the fish in its actual location. To catch it, you’d have to mentally correct for its real location, grab there, and hope for the best since fish don’t like to be caught. 

Our vision system works best when we perceive things through a medium of a single density, like the transparent air we breathe. But our eyes sometimes see scenes that have multiple mediums and densities, like air and water, at the same time. The brain tries to make sense of what our eyes see. This causes optical illusions like the bent spoon and the elusive fish. 

When the hot summer sun warms the surface of the ground, we often see a blurry mirage because the warm air has a different density than the cooler air above it. This is another natural example of refraction. 

Air and water are both transparent, but air is a lot less dense than water. Imagine trying to ride a bicycle down a flooded street versus a dry sidewalk. The densities of air and water tend to produce dramatically different experiences. 

Refractive index is a mathematical way to capture these differences in density. The index compares every other density to that of the vacuum of space, which is represented as 1.0. By comparison, air has a refractive index of 1.0003, while water is rated at 1.333 and diamonds are listed at 2.417.

Refractive index is much more than a mathematical comparison. It also helps eye doctors find the right lenses to fix refractive errors and clear up blurry vision. Moreover, high-index eyeglass lenses are manufactured to provide maximum refraction in a thin, lightweight form that is more comfortable and attractive.  

Scientists have understood the potential of refraction for centuries. From the early 1500s through the present day, telescopes and microscopes have been giving scientists a window on distant planets and living things invisible to the naked eye.  

Thus, refraction has helped humans build on sophisticated natural vision systems that evolved over hundreds of millions of years. In our species, light waves refract first in the cornea, the clear half-dome-shaped tissue on the front of the eye. After that, they pass through the lens, which refracts them again to produce clear vision.

Next, light waves go to the retina, a layer of light-sensitive nerves that transmit light sensations over the optic nerves to the brain’s visual centers, which finish the job of giving us eyesight. 

Alas, evolution has not yielded a perfect eyeball design. 

Subtle differences in the shape of the eye cause refractive errors that produce nearsightedness, farsightedness and astigmatism. Fortunately, placing another lens in front of the eye, with eyeglasses or contact lenses, adds another refraction that usually corrects these errors. 

The best way to make refraction work for you is to make regular visits to an eye doctor, who can test your vision and find the right way to correct refractive errors. 

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