GCSE Physics - How Lenses Work - Convex & Concave Lenses | Real & Virtual Images (2026/27 exams)

There are two main types of lenses: convex and concave. Convex lenses have an outward bulge, causing them to converge parallel light rays to a single point called the principal focus. Concave lenses, conversely, cave inwards, causing them to disperse parallel light rays outwards.

All lenses have a principal focus on both sides, equidistant from the center. This principal focus may also be called the focal point. Lenses are symmetrical and work both ways. The principal focus always sits on the axis, which is the line through the middle of the lens. The distance between the principal focus and the center of the lens is the focal length. A shorter focal length indicates a more powerful lens that refracts light more strongly.

To make a lens more powerful, its curvature needs to be increased, or a material that refracts light more strongly should be used.

Images are formed where light rays appear to converge. A real image is formed when light rays actually converge to a point, and it can be captured on a screen. An example is the image formed on the retina of the eye, which is inverted. A virtual image is formed when light rays do not actually converge, but rather appear to diverge from a point; these images cannot be projected onto a screen. A mirror image is a common example of a virtual image.

When describing images in exams, it's important to state whether they are real or virtual, upright or inverted, and larger or smaller than the original object. For instance, a real image is often inverted and smaller, while a virtual image is typically upright and smaller.