image distance

Meaning: Image distance (i) is the distance from a mirror or lens to its image, measured along the principal axis. Its sign gives the image type. Positive i means a real image, where light actually converges: in front of a mirror or on the far side of a lens. Negative i means a virtual image, where rays only appear to diverge from: behind a mirror or on the object's side of a lens.

Equation: 1/f = 1/o + 1/i, where f is focal length (m), o is object distance (m), and i is image distance (m). Magnification is m = −i/o (unitless); a negative m means an inverted image.

On the MCAT: Given f and o, solve for i, then use its sign and m to call the image real or virtual, upright or inverted, enlarged or reduced. A converging lens or concave mirror gives positive i when o > f; for o < f, i is negative (upright, enlarged, like a magnifying glass). Diverging lenses and convex mirrors have negative f and give negative i for a real object.

Common confusion: Students tie the sign to a side. Positive i means real for both mirrors and lenses, though the real image sits on different sides in the two cases. Also, i is measured from the mirror or lens itself, not from the focal point.