Optics and refraction
Refraction in convex and concave spheres
A convex lens (plus lens) is like two prisms placed base to base. Light passing through a convex lens is converged. Convex lenses are used to treat presbyopia, hypermetropia and aphakia. If parallel light is brought to a focus at 1 metre the lens is said to have 1 DIOPTRE of power. If the focus is at 1/2 metre, 2 DIOPTRES, and a 1/3 metre, 3 DIOPTRES (Figure 1).
A concave lens (minus lens) is like two prisms placed apex to apex. Light passing through a concave lens is diverged. Concave lenses are used to treat myopia (Figure 2).
Refraction of the eye
Light entering the eye is converged at the cornea and then the lens acts as a focusing mechanism to converge the light to a point on the retina, so that the object is seen clearly (Figure 3).
If the object is close to the eye then the lens changes shape so that the light rays can still be focused on the retina. This is called ACCOMMODATION. (The ability to accommodate begins to fail after the age of 40, so that spectacles are needed for close work.) (Figure 4)
In myopia light rays are focused in front of the retina, so that a minus concave lens is needed to diverge the rays (Figure 5).
In hypermetropia, it is the opposite, the rays are focused behind the retina so that a plus convex lens is needed (Figure 6).
Refraction in cylinders
A cylindrical lens has different powers in the vertical and horizontal axes. Thus, light passing through a cylindrical lens does not focus at one point, but forms two foci, one for the horizontal and the other for the vertical. Cylindrical lenses may be convex, concave or mixed. They are used to treat astigmatism.
Illustrations and Text Reproduced by Kind Permission From: Hanyane: A Village Struggles for Eye Health
Erika Sutter, Allen Foster, Victoria Francis
Published by the International Centre for Eye Health. (Also available in French)
Available from: International Resource Centre, ICEH, 11-43 Bath St., London, EC1V 9EL
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