I’ve had several conversations with a number of my friends concerning the wearing of glasses; some of them don’t want to wear them because they “look bad” or because they don’t want to look “nerdy”, while others think that their vision is “good enough.” Regardless, the eye is a fascinating organ, and the reason for glasses is something many people don’t actually understand. Personally, I like wearing glasses; they make me look smart, and my visual ego will take all the stroking it can receive.
Anyway, to understand the eye, one must know about the (arguably) exciting branch of physics known as Optics. Optics deals with light–its properties, behavior, etc.; since the eye is just a light sensitive organ, the two go hand in hand. The eye is actually like a camera for our brain. Light passes through a lens located at the front of the eye, and (ideally) converges the incoming light at the retina. The retina serves the same purpose as the film of a camera (or for a digital camera, a CCD chip): light hits the retina, creating a 2-D version of the world around us, which cause electrical impulses to be sent to the brain via the optic nerve. The brain interprets those impulses and creates the world that we see. With one eye, it would look just like a camera’s image, with the depth being inferred by our knowledge of the world, but not actually seen visibly in the photograph. However, because we have two eyes, our brain is constantly receiving two different images at slightly different angles, and can integrate the two to create a 3-D world. Thus, people with one eye, or who cover one eye long enough time will have severe problems with depth perception (so stop blaming your bad driving on your terrible depth perception — you have no excuse!).
All lenses, including those in your eye, bend light in ways depending on the shape. The lenses in your eye are convex, shaped a little like a small cigar. Convex lenses, also known as converging lenses, take incoming light and converge it at what’s known as a focal point, the place where all light beams meet. It is at this focal point that a focused image is created. If you look at an object through a converging lens, unless the image screen is placed at the focal point, the picture created will be blurry. We’re now approaching how you explain Myopia (nearsightedness) and Hyperopia (farsightedness). The lens of the eye has to converge light from the world onto your retina. It cannot focus the entire world at once (this is easy to notice when you focus on a far away object, and then immediately focus on, say, your hand–your eye has to increase its focusing power to see your hand clearly), and so the eye adjusts the lens to allow you to focus on the image at which you are looking at. However, sometimes, it is unable to focus on either close objects (Hyperopia) or far away objects (Myopia). If the eye’s lens is unable to focus the light on your retina, the images become blurry. In the case of farsightedness, you cannot focus on nearby objects, because the image converges behind your retina. So how do you correct it? You wear glasses, or “corrective lenses”; these lenses are converging, and shift the focal point to your retina.
So that explains farsightedness and its corrective lens solution…but what about nearsightedness? Well, people with Myopia have the opposite problem — when trying to focus on far away objects, the focal point of the lens falls short of the retina, and needs to be shifted backwards. This is done using a concave or diverging lens. This type of lens is shaped like an hourglass, and causes light beams to be scattered away from each other. Then, the light beams get converged by the eye’s lens, and voilà, the focal point is pushed back onto the retina. Focused images!
I have (hopefully) demonstrated the cause of bad eyesight, and why glasses are a necessity for many people. Of course, contact lenses work too, it’s just that they are directly on top of the eye, hence contact lenses — the correction without sacrificing the “cool.” I don’t know, I’m a four-eyes and proud of it. So if you have bad eyesight, please get some glasses! I’m looking at you, my nameless friends.
This entry was originally featured on Science for Dessert, a blog created by Dan Feldman. For more cool science, visit his site.
I wonder how do Bifocal lenses work then?
And won’t they harm your eyes because there are two different lens in them?
Bifocal lenses are used when people suffer from both Hyperopia and Myopia. So part of the glasses will have a convex lens, and the other has a concave lens—this way, you don’t have to have 2 separate pairs of glasses for different types of vision. But it’s true that you can get headaches and dizziness from not being used to the way bifocals are designed to be utilized. There are a whole bunch of different ways bifocal lenses have been designed, but they all follow these same principles—you bend the light so it will focus on your retina, allowing vision to be clear and not blurry!