Myopia, or "near-sightedness" is a very common ocular condition typically first found in school-age children.  These individuals have good near vision, but blurry distance vision.  They will typically need glasses for full time wear, especially for activities like driving.  Let's do a little break down of what causes myopes to have this blurry distance vision and how glasses work to correct the issue:

--Myopia is a result of one (or more) structural variations of the shape of the eye.  Each element of the eye has a certain size and shape to be "within normal limits," but when these measurements fall  slightly outside of normal, a refractive condition results.  Items that specifically affect the power of the eye are:

• The Cornea (a thicker than normal cornea will result in myopia)  Note: this is why surgeries like PRK and LASIK work to correct a myope's vision; these techniques reduce corneal thickness!
• The Lens (typically the lens thickness has minimal impact, but as it changes in conditions like cataracts, myopia may result)
• The Axial Length (this is the length of the eye; if the eye is too long, myopia will result)

Courtesy the National Eye Institute

How do glasses help myopia?  Optometrists use lenses in front of the eye to change how much light is bent (or refracted) as it travels through the eye so that a clear and distinct image falls on the retinal cells.  For myopia, "minus" lenses are used, which are thicker on the outer edges of the lens and thinner in the center.  With today's lens cutting technology, this difference in thickness if usually very subtle, so you may not notice it if you are a myope and just took off your glasses to test me out, haha.  Lenses have varying thickness and are measured in "Diopters (D)."  The dioptric value of the lens you need is the exact match to the amout of myopia you have (or the amount the image needs to be adjusted in order to fall clearly on the retina).  For example, I wear a -2.00 D lens, which corrects my 2.00 diopters of myopia.

Now what does this measurement of myopia mean?  To continue my example, a 2.00D myope means that I can't see clearly at any distance past 50 cm in front of my face.  Hold on -- how did I figure that out?  It is luckily a very simple equation.  If you divide 100 cm (1 meter) by 2.00D, you get 50 cm as the far point at which I can still see clearly.  This works for any amount of prescription.  If you are a 4.00D myope, then you can't see clearly past 25 cm in front of your face.  If you are a 20.00 D myope, well you get the picture, you have a hard time seeing much of anything.

Minus glasses not only clear the image to their wearers, they can cause a minification effect (especially in the higher powers).  This is why when you see someone wearing glasses for myopia it may look like their eyes are very small within their glasses (see picture below).

Note how the side of Buddy Holly's face seems smaller inside of his glasses.   Today, we can use high index material lenses to reduce the thickness of the lens and this minification effect.