concave lenses

Meaning: A concave lens, also called a diverging lens, bends incoming parallel rays away from the axis. The refracted rays do not meet; they appear to come from a focal point on the incident side of the lens. That focal point is virtual, so by the MCAT sign convention f is negative and the power is negative.

For a real object (o positive), the image is virtual (i negative), upright, and smaller than the object. It lies between the lens and the focal point on the object's side. Moving the object closer or farther changes the image size and position, but not these features.

Equation: 1/f = 1/o + 1/i, and m = −i/o, where f is focal length (m), o is object distance (m), i is image distance (m), and m is magnification (unitless). Power P = 1/f, in diopters (m⁻¹). For a concave lens f < 0, so P < 0.

On the MCAT: Passages ask for the image type from a ray diagram, or give o and f and ask for i or m. Expect a negative i, and |m| < 1 with m positive. Vision-correction questions pair concave lenses with myopia (nearsightedness), because the lens diverges light before the eye focuses it.

Common confusion: Students mix up concave lenses with concave mirrors. A concave mirror converges light and has positive f. A concave lens diverges light and has negative f. Check the sign of f before using the lens equation.