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The Physics of Contact Lenses

*The left figure shows a nearsighted lens while the right figure shows a farsighted lens.

A nearsightedness lens has a lens that is thinner in the center than on the edges (concave) increasing the focal length, this lens is called a diverging lens [see Negative(diverging lens)]. A farsightedness lens is thicker in the center than on the edges (convex) decreasing the focal length, this lens is called a converging lens [see Positive (converging lens)]. Lenses are custom shaped to fit irregular shapes of eyes that suffer Astigmatism. A lens for presbyopia has two or more shapes or curves combined into one lens. The bottom part of the lens would be used for reading while the top part would be for looking at objects in at a distance.

The value of the focal length f for a particular lens can be calculated from the lensmaker's equation:

$\frac{1}{f} = (n-1) \left\{ \frac{1}{R_1} - \frac{1}{R_2} + \frac{(n-1)d}{n R_1 R_2} \right\},$

*n is the refractive index of a material at a particular frequency is the point where the electromagnetic radiation of that frequency is slowed down (relative to vacuum) when it travels inside the material. If the material is not magnetic that the refractive index is the materials dielectric constant[ relative permettivity multiplied (εr) by the permettivity of free space (μ)].

$n=\sqrt{\epsilon\mu}$

* d is the distance along the lens axis between the two surfaces (thickness of lens). If d is small compared to R1 and R2 then the equation can be simplified.

$\frac{1}{f} = (n-1)\left\{ \frac{1}{R_1} - \frac{1}{R_2} \right\}.$

If the focal length is positive then the lens is converging and opposite if lens is diverging. The value 1/f is know as the power of the lens.

Materials

The early contact lenses where made from hard plastic that allowed little breathing room and forced the eye to dry up easily and they would sometimes pop out of the eye being very uncomfortable. Newer contact lenses are made from soft plastic that allows the eye to breathe. They are more comfortable because of their high water content material. These soft contacts cling to the layer of tears covering the cornea and move slightly with each blink. This allows the eye to better stay moist and wash out debris.