Why Reading Glasses Always Have a Plus Power
If you have ever shopped for reading glasses, you know that the power of the lenses are always specified with a plus sign in front of the number (+). This plus sign means the lenses are converging light rays to bring images in focus closer than they really do.
Lets explain that a little further. In general, consider there a three types of lenses. Concave surfaced (minus), flat surface (plano), and Convex surface (plus).
A lens with a concave surface will expand light rays as they pass through, bringing the area of focus further away. In reality, if you expand light rays, they will never meet until they go through another (convex) refractive surface. Nearsighted (myopic) eyes require minus lenses to bring the focal area of the image further back in to the eye.
A truly flat surface lens will have an overall power of zero, meaning it does not refract light. Think of a window, you can see through it, but it does not change the image. Have you ever looked through an old glass window that is not 100% smooth? Notice how the image gets distorted? Consider those wrinkles as little lenses at work.
Now, back to reading glasses; the reason we need reading glasses is to re-adjust the focal point closer now that our eyes are no longer capable of doing so on their own. So, a plus power, convex lens is just what we need. These lenses will bring our range of focus closer with each diopter we add. (a diopter is the measure of focal length. A +2.50 pair of reading glasses has 2 and Ĺ diopters of plus power.)
With each increase in diopter strength, the bigger an object may appear, but the closer we have to hold it to see it. So a +1.00 pair of reading glasses may give you a clear image of the computer screen, but may not be adequate for tiny hobby activities. And a +3.00 may look great for removing a splinter, but you can forget about comfortably reading a newspaper.
The actual power you need is going to be more a function of age, visual demands, and existing vision correction.
If you need reading glasses and tried to put on a pair of glasses with a minus power lens, it would simply exasperate the problem. On the same note, if you have a friend that is in their late thirties and wants to experience what it is like to need reading glasses, all they have to do is slip on a pair of -2.00 eyeglasses and try to read a magazine.
All the optics aside, it comes down to simple algebra. Thank goodness when it comes to reading glasses we usually donít have to go above a +3.00.