lens
Summary
A lens is a transparent curved object that refracts light to form an image. In air, a convex lens converges rays (f > 0) and a concave lens diverges them (f < 0). Most testable: use 1/o + 1/i = 1/f, where the sign of i tells real from virtual.
Detail
Meaning: A lens is a transparent object with curved surfaces that refracts light to form an image. In air, a converging (convex) lens is thicker at the center and bends parallel rays to a real focus, so f is positive. A diverging (concave) lens is thinner at the center and spreads parallel rays as if from a virtual focus, so f is negative.
Equation: 1/o + 1/i = 1/f and m = −i/o, where o is object distance (m), i is image distance (m), f is focal length (m), and m is magnification (unitless). Power P = 1/f, in diopters (m⁻¹).
On the MCAT: Tested with ray diagrams and thin-lens calculations. A converging lens with the object beyond f gives a real, inverted image; inside f it gives a virtual, upright, enlarged image, as in a magnifying glass. A diverging lens gives a virtual, upright, reduced image of a real object. With two lenses, the first image is the object for the second.
Common confusion: Students mix up the signs. Positive i means a real image on the far side of the lens, and it is inverted (m < 0). Negative i means a virtual image on the object's side, and it is upright (m > 0).
Sources
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
- Physics: Principles with Applications (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.