A typical magnifying glass consists of a single thin bi-convex lens that produces a modest magnification in the range of 1.5x to 30x, with the most common being about 2-4x for reading or studying rocks, stamps, coins, insects, and leaves. Magnifying glasses produce a virtual image that is magnified and upright.
This interactive tutorial demonstrates how a simple, thin bi-convex magnifying lens works to produce a magnified virtual image on the retina. In order to operate the tutorial, place the mouse curser over the bi-convex (simple magnifying) lens and hold down the left mouse button while dragging the lens up and down to vary the distance between the lens and the object (in this case, a rose). Moving the lens closer to the rose reduces the magnification, while moving the lens farther away from the rose increases the magnification. The image is perceived by the eye as if it were at a distance of 10 inches (25 centimeters) from the magnified object. The image produced by a simple magnifying lens appears to be on the same side of the lens as the object and is termed a virtual image. These images are always appear upright (not inverted) to the observer.
The distinction between a real and a virtual image is an important concept when imaging specimens through a lens or mirror system, regardless of whether the system consists of a single or multiple components. In general, images are defined by the regions where the rays (and their extensions) become convergent as the result of refraction by a lens or reflection by a mirror. In cases where the light rays intersect at a focal point, the image is real and can be viewed on a screen, recorded on film, or projected onto the surface of a sensor such as a CCD or CMOS placed in the image plane. When the light rays diverge, but project imaginary extensions that converge to a focal point, the image is virtual and cannot be viewed on a screen or recorded on film. In order to be visualized, a real image must be formed on the retina of the eye. When viewing specimens through the eyepieces of a microscope, a real image is formed on the retina, but it is actually perceived by the observer as a virtual image located approximately 10 inches (25 centimeters) in front of the eye.