converging lens

Meaning: A converging lens (convex lens) is thicker at the center than at the edges. In a surrounding medium of lower refractive index, such as air, it bends parallel incoming rays toward a focal point a distance f beyond the lens. Its focal length is positive.

Equation: 1/o + 1/i = 1/f and m = −i/o. Here o is object distance (m), i is image distance (m), f is focal length (m), and m is magnification (unitless). Lens power is P = 1/f, in diopters (m⁻¹).

On the MCAT: Questions give o and f and ask for the image type, location or size. Object beyond f: the image is real and inverted, forms on the far side of the lens, and has positive i. Object inside f: the image is virtual, upright and enlarged, forms on the object's side, and has negative i, as in a magnifying glass. At o = f, no image forms because the refracted rays emerge parallel. Questions on the eye or corrective lenses use the same rules.

Common confusion: Students assume a converging lens makes a real image. It does so only when o > f. A concave mirror also converges light, but its real image forms on the same side as the object, while a lens forms its real image on the opposite side.