A convex lens is a transparent optical device that is thicker in the center and thinner at the edges. This specific curvature causes incoming parallel light rays to bend inward, or refract, toward a single point, earning the device the alternate name of a converging lens. Convex lenses are fundamental components in a variety of optical instruments, including cameras, microscopes, and eyeglasses. Whether a convex lens produces a real image depends entirely on the distance of the object from the lens, as a single convex lens can produce either a real image or a virtual image.
Understanding Real vs. Virtual Images
The distinction between a real image and a virtual image is based on the actual path of light rays after they pass through the lens. A real image is formed at a location where the light rays originating from the object physically converge and cross one another. Because the light energy is physically focused at this point, a real image can be captured and displayed on a physical surface, such as a screen or film. Real images formed by a single lens are always inverted relative to the original object.
A virtual image, in contrast, is formed where the light rays only appear to have diverged from a point, but they do not actually meet at that location. Since there is no physical convergence of light, a virtual image cannot be projected onto a screen. It is only perceptible to an observer looking through the lens. Virtual images are always upright, maintaining the same orientation as the object.
When Convex Lenses Form Real Images
A convex lens will produce a real image when the object is positioned outside of the lens’s focal point (\(F\)). The focal point is the specific distance from the lens where parallel rays of light converge. The object must be placed at a distance greater than this focal length for a real image to form. When this condition is met, the light rays from the object pass through the lens and physically cross on the opposite side of the lens, creating the real image.
The characteristics of the resulting real image are determined by the object’s precise distance from the lens, especially in relation to the point \(2F\), which is twice the focal length. If the object is placed beyond \(2F\), the image is formed between \(F\) and \(2F\) and is smaller and inverted. Conversely, if the object is placed between \(F\) and \(2F\), the image is formed beyond \(2F\) and is larger and inverted. An object placed precisely at \(2F\) will produce an image that is also at \(2F\) on the opposite side, having the same size as the object.
What Makes a Convex Lens Form a Virtual Image
A convex lens produces a virtual image when the object is placed closer to the lens than the focal point (\(F\)). When the object distance is less than the focal length, the light rays passing through the lens still refract toward the principal axis, but they do not converge quickly enough to cross. Instead, the light rays diverge as they exit the lens, spreading apart on the side opposite the object.
The eye and brain trace these diverging rays backward to a point on the same side of the lens as the object, forming the virtual image. This image is always upright and magnified. The magnifying glass operates on this principle, as the user holds the lens close to the object, placing it within the lens’s focal length to produce an enlarged, upright virtual image.