corrective lenses

Meaning: Corrective lenses are placed in front of the eye so light from an object focuses on the retina when the eye alone cannot do this. In myopia (nearsightedness) the image forms in front of the retina, so a diverging (concave) lens with negative focal length is used. In hyperopia (farsightedness) the image forms behind the retina, so a converging (convex) lens with positive focal length is used. The lens forms a virtual image at a distance the eye can focus on: the far point in myopia, the near point in hyperopia.

Equation: P = 1/f = 1/do + 1/di, where P is lens power in diopters (m⁻¹), f is focal length (m), do is object distance (m), and di is image distance (m), negative for a virtual image.

On the MCAT: A passage gives a far point or near point and asks for the lens power or type. Myopia: the object is at infinity, so di = −(far point) and f = −(far point). Hyperopia: the object is at 25 cm, the usual normal near point, and di = −(the person's near point).

Common confusion: A nearsighted person sees near objects clearly, yet needs a diverging lens with negative power. The sign comes from the virtual image (di < 0), not from the name of the defect.