The Core Principle: Refraction
Lenses focus light primarily through refraction, the bending of light as it passes from one transparent medium (like air) into another (like glass or plastic) and then back again. This bending occurs because light changes speed when entering a new medium, causing its path to alter. The precise shape of the lens dictates how these light rays are redirected to either converge or diverge.
Converging Lenses (Convex Lenses)
A convex lens, which is thicker in the middle and thinner at the edges, causes parallel light rays to converge. As light enters the curved convex surface, it bends inward towards the normal. Upon exiting the second curved surface, it bends further inward, effectively gathering all parallel incoming rays to a single point called the focal point on the opposite side of the lens.
Diverging Lenses (Concave Lenses)
In contrast, a concave lens, thinner in the middle and thicker at the edges, causes parallel light rays to diverge, or spread out. Each ray entering a concave lens bends outward away from the normal. This makes the rays appear to originate from a single virtual focal point located on the same side of the lens as the incoming light.
Applications of Lens Focusing
The ability of lenses to focus light is crucial for countless optical instruments and biological systems. Eyeglasses and contact lenses correct vision by focusing light accurately onto the retina. Cameras use multiple lenses to focus light onto sensors, while telescopes and microscopes utilize complex lens systems to form clear, magnified images of distant or tiny objects, respectively.