concave mirror
Summary
A concave mirror curves inward like the inside of a spoon and converges light to a focal point at f = r/2, so f is positive. Beyond f its image is real and inverted; inside f it is virtual, upright and enlarged.
Detail
Meaning: A concave mirror converges parallel rays (near the axis) to a focal point in front of it at f = r/2. The image depends on the object's position. Beyond the center of curvature C: real, inverted, reduced. Between C and f: real, inverted, enlarged. Inside f: virtual, upright, enlarged. At f, no image forms because the reflected rays are parallel.
Equation: 1/o + 1/i = 1/f = 2/r, with m = −i/o. Here o is object distance (m), i is image distance (m), f is focal length (m), r is radius of curvature (m), and m is magnification (unitless). Positive i is a real image in front of the mirror; negative i is virtual, behind it.
On the MCAT: Given o and f, solve for i and m, then use the signs to classify the image. Ray diagrams use a ray parallel to the axis (reflects through f) and a ray through f (reflects parallel). Makeup mirrors and reflecting telescopes are typical examples.
Common confusion: Students mix it up with a concave lens, which diverges light, or assume a concave mirror gives a virtual image. The sign of i settles it: positive i means real and inverted, negative i means virtual and upright. A convex mirror is the one with negative f.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway and Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.