image formation
Summary
Image formation is how a mirror or lens redirects light from an object so the rays either meet (real image) or only appear to diverge from a point (virtual image). Most testable: 1/f = 1/o + 1/i gives image distance, and the sign of i tells you real (positive) or virtual (negative).
Detail
Meaning: A mirror or lens redirects rays from an object. If the rays actually meet, the image is real: it can be projected on a screen and is inverted. If the rays only appear to diverge from a point, the image is virtual: upright and not projectable. Converging elements (concave mirrors, convex lenses) can form either kind. Diverging elements (convex mirrors, concave lenses) form virtual, upright, reduced images of real objects.
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). For a spherical mirror f = R/2, with R the radius of curvature (m).
On the MCAT: Given f and o, find i and M, or predict how the image changes as the object moves. For a converging element, an object beyond 2f gives a real, inverted, reduced image; between f and 2f, real, inverted, enlarged; inside f, virtual, upright, enlarged. With two lenses, the first image is the object for the second.
Common confusion: Positive i means real, but the side differs: in front of a mirror, beyond a lens. Negative M means inverted, not smaller; size is given by |M|. Virtual images can still be seen by the eye, just not projected.
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.