How Things Work: Contact Lenses
Many people find that wearing eyeglasses is a hassle, especially when they need them for various activities. How is it that a pair of eyeglasses can have the same effect on blurry vision as a pair of thin contact lenses? And in addition to wearing contacts to correct vision, some people wear contacts to change the color of their eyes.
The first contact lenses were water-filled glass tubes attached to tiny lenses — not the most comfortable things to wear at all. They covered the entire eyeball and were so heavy that they could only be worn for a few hours.
Contact lenses give wearers a more natural field of vision because they move with the eyes and are able to correct the refractive error closer to the eye. The lenses stick to a layer of tear fluid on the surface of the eye. They are also held in place by eyelid pressure and by blinking, which lubricates the cornea and flushes away any debris that accumulates on the lenses.
In normal vision, light rays enter the eye through the cornea and become focused on the retina at a central point in the back of the eye called the focal point. Nearsightedness and farsightedness result from light not being focused directly on the retina. The shape of the eye is very important in keeping what we see in focus. Depending on what type of vision problem a person has, the shape of the contact lense helps refocus light on the retina. Contact lenses work in much the same way as eyeglasses.
People with nearsightedness can see things clearly close up, but can’t see things far away. Nearsightedness is the result of the eyeball being too long, causing light rays to focus in front of the retina instead of on it. The contact lenses needed to correct nearsightedness are concave: They’re thinner at the center than at the edges. This helps to spread the light throughout the lenses and focuses light back onto the focal point of the retina.
Meanwhile, people with farsightedness can see clearly far away, but have trouble seeing things up close. It is the result of the eyeball being too short, which causes light to focus behind the retina. The contact lenses needed to correct farsightedness are convex. They’re thicker at the center, which bends the light toward to center and moves the focal point, which is behind the retina, to the retina.
To correct astigmatism, a special type of lens called a toric lens is required. Astigmatism occurs when the cornea is irregularly shaped, which causes light to focus at several points. These lenses are made specifically for each individual to correct the different angled curvatures. The lenses must be put in the eyes in a specific way. They usually have more weight on the bottom or have a thinner top and bottom edge to keep them fixed in one position on the eye.
Color contact lenses not only correct vision, but they also enhance or change eye color. Eye color is determined by how much melanin, a dark brown pigment, is at the surface of the iris.
Opaque lenses have solid color applied to the surface of the lens in a variety of shapes and patterns that resembles the natural look of the eye. When worn, the solid colors cover the natural color of your eyes. Your natural eye color, showing through the clear spaces, blends with the color being reflected from the solid colors.
Tinted lenses work well on light-colored eyes, but not on dark-colored eyes. Light reflects off the lens and passes through the lens. The light-colored eye’s iris absorbs some of the light and reflects the lighter color. In dark eyes, the light that passes through the lens is absorbed and a darker color is reflected back.
The strength of the lens, expressed in diopters, depends on how much it bends light. The higher the diopter, the stronger the lens.
Contact lenses are similar to glasses, but more care must be taken with contacts. Problems with contact lenses can include discomfort, itchiness, or redness — possibly due to a lack of oxygen-carrying tears. Even though these thin transparent plastic discs are fragile, they work wonders with blurry vision!