Week 8: Curvature of the Lens

Around 75% of Americans use corrective lenses to aid in fixing refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia. With all of these refractive orders being unique to each individual, each corrective lens is custom made to fix the refractive error. This is the biggest reason for people with a astigmatism and myopia can not switch glasses with another individual. How are these lenses made to bend light so that the focal point is near the fovea of the eye?

Concave Lens

Myopia is corrected with what is known as a concave lens, which bends light outwards that comes through the lens. This lens is also known as a divergent lens since it spread out light rays. Examples of it's uses other than eyeglasses are binoculars, telescopes, and microscopes (these three objects also use convex lenses also). Examples of objects that are purely concave are flashlights, lasers, and peepholes. The interesting part about the flashlight is that the convex lens allows for those diverging rays so that the radius of the light is increased.

Convex Lens

Hyperopia and presbyopia are corrected with a lens that curves which is called a convex lens or a converging lens. The light is refracted so that rays bend inwards. This type of lens allows for magnification to have objects appear closer than they appear. Examples of this lenses use would be microscopes, telescopes, cameras, and binoculars.

Astigmatism

Astigmatism is corrected by a lens that have different radii of curvature to correct for the refractive error. This simply means that the lens could be convex in one place and concave in another to take multiple focal points into one on the back of the eye. This disorder is due to an incorrect curvature of the lens or cornea.

Glasses are not just designed according to the lens shapes that are mentioned above. Lenses are made using convex and concave shapes which is known as a ophthalmic lens/glasses. The front part of the glasses  that is further away from your eye has the convex shape. The back of the lens (one closer to the eye) has the concave shape. Its the difference in curvature between the from and the back the lens that provides the corrective power.

Blackboard Optics - Concave and Convex Lenses from Michael Thorpe on Vimeo.

Sources:

• Wikipedia: Lens Profile
• American Optometric Association: Astigmatism
• eHow: Concave Lens Uses
• eHow: Convex Lens Uses
• Universe Today: Concave Lenses