virtual image

A virtual image is formed when light rays, after passing through a lens or reflecting from a mirror, appear to diverge from a point behind the optical surface, even though no light actually converges there. Unlike a real image, which can be projected onto a screen, a virtual image exists only as a visual illusion. You see it because your eye traces the diverging rays backward along straight lines until they meet at an imaginary location.

The most common source of virtual images is the convex mirror, familiar as a security mirror or car side mirror. Light reflects outward in all directions, and when extended backward through the mirror surface, these rays seem to originate from a point behind the reflective surface. The image appears smaller, upright, and farther away than it actually is. Similarly, a concave lens (minus lens) with negative focal length always produces a virtual image on the same side of the lens as the object being viewed. This is why reading glasses for myopia create virtual images; the diverging light rays make the printed page appear farther away and smaller than it would be to an uncorrected eye.

Formation and optical characteristics

Virtual images form at distances governed by the lens or mirror equation: 1/f equals 1/d_o plus 1/d_i, where f is focal length, d_o is object distance, and d_i is image distance. When d_i comes out negative in the calculation, the image is virtual. The magnification of a virtual image is positive, meaning the image orientation matches the object. For a plane mirror (infinite focal length), the virtual image forms at a distance behind the mirror equal to the object distance in front of it, always at unity magnification.

Virtual images are essential in optical instruments where magnification or field expansion is needed without projection capability. Magnifying glasses, loupe lenses, and jeweler's viewers all rely on virtual images to enlarge detail for the human eye. Periscopes and astronomical telescopes often incorporate virtual image formation in their eyepiece stages. The term 'virtual' distinguishes these from real images, which converge light to a physical point and can be captured on a detector or screen.

In practice, optical systems must account for virtual image behavior when designing magnification, field of view, or aberration correction. Measurement of virtual image position requires tracing rays through the optical system or using optical simulation software, since no screen can intercept a virtual image. Understanding where virtual images form is critical for troubleshooting magnification complaints or designing eyepieces and viewing optics.