The figure below shows the structure of the human eye. As you can see, light rays pass through the cornea (outer covering) and lens and are focused by these two structures on the retina at the back of the eye. You might notice that the image is upside down; it is also reversed from left to right. Your brain automatically compensates for these effects.

What do you suppose happens if your eyeball is too long, so that the light rays are actually focused somewhere in front of your retina? Or, what happens if your eye is too short, and the light rays are focused behind your retina? In either case, the image you see would be blurry and out-of-focus. You will likely go to an eye-care professional and obtain an artificial lens to focus the light rays on your retina. In other words, you’ll either wear glasses or contact lenses.