# Human Eye

By Levi Clancy for Student Reader on
updated

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TermUnitsDescription

Near Point (NP, N)

meters (m)

Closest distance the eye can focus clearly. A normal eye has a near point of 25 centimeters. For a relaxed eye the near point is ∞.

Far Point (FP)

meters (m)

Farthest distance the eye can focus clearly. A normal eye has a far point of ∞.

Myopia

Myopia (nearsightedness) occurs when the focal point is in front of the retina. A correcting lens is a diverging lens.

Hyperopia

Hyperopia (farsightedness) occurs when the near point is greater than 25 cm from the eye. A correcting lens is a converging lens.

## Corrective Lenses

TermUnitsDescription

Power

Diopter (D)

The power of a corrective lens is F-1 and thus has units m-1 (diopters). A negative Power means nearsightedness, while a positive Power means farsightedness.

VisionDescription

Farsighted

Corrective Power = (Actual N.P.)-1 - (Normal N.P.)-1
The normal eye near point is .25 meters.

Nearsighted

Corrective Power = - (Actual Far Point)-1
The normal eye far point is ∞ and 1/∞ is zero.

Eye-Lens Distance

dx

This is the distance from eye to corrective lens. Many assignments disregard the distance between the eye and corrective eyeglass lenses, and assume it to be zero. If this is not disregarded, then just subtract this distance from the near/far points. The distance between the eye and corrective contact lenses is of course zero.

## Magnifying Glasses

###### Magnification (M)

M = θ' / θ
M = 1 + ( di / F ) = NP / do

###### Using magnifying glass: object placed much closer to the eye, and image at near point.

Magnifying glass Magnifying Power M = θ' / θ = N.P. / do
θ = h / N.P.
θ' = h / do