Thin lens equation
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
1/f = 1/d_o + 1/d_i; magnification m = -d_i/d_o = h_i/h_o; converging lens f > 0, diverging f < 0.
Thin lens equation in plain English
A lens bends light to form an image, and this one equation predicts exactly where that image lands and how big it is. In 1/f = 1/d_o + 1/d_i, f is the lens's focusing strength (its focal length), d_o is how far the object sits from the lens, and d_i is where the image forms — so if you know two of them, you solve for the third. The magnification m = -d_i/d_o tells you the image's size and orientation: a negative m means the image is flipped upside down, which is exactly what a magnifying glass does when you hold it far from a page. A converging lens (fatter in the middle, like that magnifier) has f > 0 and can focus light to a point; a diverging lens has f < 0 and spreads light out instead.
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