If you do not focus well on objects in the distance, your eye’s lens brings the light rays that make up the image together before they reach the retina- this is known as myopia. On the other hand, if you cannot focus well on near objects or writing (hyperopia), the eye’s lens brings the light rays together at a point beyond the retina.
Many centuries ago, it was discovered that a glass prism could bend light. The bending of light is the basic principle behind how corrective glasses lenses work. When you are given a prescription for eyeglasses, it is either a positive or a negative number. Positive numbers refer to prescriptions that correct hyperopia, while negative prescriptions correct myopia.
Positive lenses (convex lenses) bring the image into focus at a focal point closer to the lens of the eye. This means that the rays of light are brought together at a point before the retina, rather than behind it. By refining the prescription, the focal point can be altered until the light rays are brought together on the retina in the correct way, giving you much improved near-sight vision.
Negative lenses, also known as concave lenses, prevent the rays of light focussing at a point before the retina. With the correct prescription, the concave lenses ensure that the light rays have a focal point just on the retina, to give you the best possible distance vision.
As you age, the strength of the muscles in your eyes that control your eye’s own lens weaken, and your prescription may change with time. A condition known as astigmatism can occur when you struggle to focus on both near and far objects. For these reasons it is important that you attend regular appointments with your optician at least once every two years.