virtual images
Summary
A virtual image is a point from which light rays only appear to diverge; no light actually passes through it, so it cannot be projected on a screen. Testable point: a virtual image has a negative image distance (i < 0) in the standard sign convention.
Detail
Meaning: The eye traces diverging rays backward to a point where they seem to originate, but no light actually reaches that point. For a mirror the image is behind the mirror. For a lens it is on the same side as the object.
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). A negative i means the image is virtual. A positive m means upright.
On the MCAT: A plane mirror forms a virtual, upright, same-size image of a real object, as far behind the mirror as the object is in front. Convex mirrors and diverging lenses form virtual, upright, reduced images of a real object. Concave mirrors and converging lenses form a virtual, upright, enlarged image only when the object is inside the focal length (o < f), as in a magnifying glass. Questions ask you to find the sign of i and describe the image.
Common confusion: Virtual does not mean unseeable. You see virtual images in a mirror daily, because the eye's lens focuses the diverging rays onto the retina. What defines a virtual image is that rays do not truly meet there, so a screen placed at that spot shows nothing.
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.