by Whitney Bell
Recently, I had to stop wearing my contact lenses for a week when North Carolina’s tree pollen season was at its peak. Like many, I suffer from seasonal allergies and my eyes were irritated by the pollen in the air. My body’s immune reaction to it included eye redness, itching, watering, and overall discomfort when wearing my contacts. You can read more about why that happens here. I gave my eyes a break and wore my glasses while the yellow dust filled the air, but I started thinking about my contact lenses and my prescription. I got my first pair of glasses in the 4th grade and eventually switched to contact lenses in high school. Here, I want to examine why so many people need corrective lenses for their vision.
Parts of Eye
We first need to understand the eye and how our sense of sight works. The cornea is the clear first layer of the eye that light passes through. Light then enters the eye through the pupil, a hole in the eye that is surrounded by the colored iris. The iris is made of muscles that constrict or dilate around the pupil, thus controlling how much light enters the eye. This is why in low light conditions, your pupils dilate; they appear bigger because the iris is reacting to allow more light to enter the eye to help you see better. Conversely, in bright light, the iris constricts, or tightens around the pupil, to limit the amount of light entering the eye. Light then passes through the lens, which works in conjunction with the cornea to focus and bend (refract) light onto the back part of the eye, the retina.
The Retina
The retina is a layer of the eye that contains millions of light sensitive cells (photoreceptors), called rods and cones, as well as nerve cells. The rods are photoreceptor cells that process light in dim conditions. The cones, however, are what give us color vision as well as sharp vision. Mutations in your cone cells cause color blindness, which you can read about here.
In the middle of the retina is the macula, and at the center of the macula is a small pit called the fovea. The fovea is responsible for sharp, clear vision because it contains only cone cells, no rods. The cells of the retina work together to process the light it has received and turn this information into electrical signals. These electrical signals are sent through the optic nerve to the brain, which then processes the signals into the visual images that we see. Pretty cool, right?
Refractive Errors
So what happens to our eyes to make us need corrective lenses like glasses or contacts? Why can’t some people see near objects and others can’t see distant objects? The most common vision problems are refractive errors, which occur when the shape of your eye prevents light from focusing correctly on that important back layer of the eye, the retina. The result is a blurred, out of focus image of an object close by or far away, depending on the imperfect eye shape.
Refraction is the bending of light as it passes through one transparent substance into another. Thus, imperfect curvature of the eye shape thus causes light to pass through the cornea and lens but focus on the wrong spot to the retina, causing out of focus images. Myopia and hyperopia, better known as nearsightedness and farsightedness, respectively, are two types of refractive errors.
Myopia
If you have myopia, you are nearsighted and can see objects close up but have difficulty seeing objects that are farther away, like street signs. This is because the altered shape of the eye focuses the image you see too far forward in front of the retina (c). The retina is where the focal plane is located in an eye with correct shape. Since the image is out of focus before it reaches the retina, the objects you see close by are in focus, but the objects far away are not in focus. In summary, if your eye focuses light in front of the retina, you are myopic and can see close objects clearly, but not far away objects.
Hyperopia
In contrast, hyperopia is when you can see objects more distant from you but have trouble seeing things close-up, like lines of text in a book. Hyperopia is due to the eye shape focusing the light too far behind the retina. As the focal plane, the point where the light is focused, is behind the retina now, the image is out of focus when it hits the retina (b). As a result, the things you see far away are in focus, but the objects near to you are out of focus. In short, if your eyes focus the light behind the retina, you are hyperopic and need help seeing nearby objects..
These vision problems are diagnosed by a special doctor that studies eyes and is called an optometrist. Myopia affects about 30% and hyperopia affects about 10% of Americans. In the illustration below, you can see that uncorrected eyes with hyperopia (b) and myopia(c) are not completely round like the normal eye (a) on the left. As previously mentioned, this slight change in eye shape impacts the way the light passes through the layers of the eye to focus on the retina. The solution to refractive errors caused by imperfect eye shape are correct lenses.
Corrective lenses
Corrective lenses, such as glasses or contacts, take the shape of your eye into account and adjust the light so it is focused and directed to the correct spot on your retina for a clear image. These lenses are created unique to the curves of your eyes, thus you have a special prescription.
If you have trouble seeing and think you may have one of these refractive errors, you should make an appointment with an optometrist. The optometrist will test and evaluate your vision using an eye chart (those rows of letters in various sizes) and calculate the prescription (the curvature of your eyes and the resulting type of corrective lens needed to fix your vision) so that you can see normally. Glasses and contact lenses made a huge difference in my life, and I’m so glad we have the science and technology that helps us see clearly!
