A lens has two surfaces through which light passes. A lens with one surface convex and one concave is called meniscus . Rays from a point source diverge from that point. Rays from a common point are called a bundle . When such a bundle enters a lens, each ray is refracted on passing through a surface.
Refraction changes the direction of the ray.
Because of this, the rays of the bundle may emerge from the lens either more divergent or less divergent, depending on the nature of the lens. Some lenses change the direction of the rays enough to cause the rays in a bundle to emerge convergent, that is, converging toward a common point. This is the most well-known situation. If the incident rays come from a point source of light located at least one focal length from a converging lens, they emerge from that lens convergent to a point at least a focal length distant from the lens. We call a point source of light a real object , and the point of convergence of the bundle of rays emergent from the lens is a real image of that object.