How Lenses Bend and Focus Light

Light changes speed when it passes from one transparent material into another. If it enters at an angle, the change in speed also changes its direction. This bending is called refraction.

A lens has curved surfaces, so rays strike different parts at different angles. The two surfaces work together to redirect the rays in an organised pattern. The exact path depends on the lens shape, the materials involved and the colour of the light.

A ray travelling through the optical centre of a thin lens changes direction very little in a simple diagram. Rays parallel to the main axis behave more dramatically. Their paths reveal whether the lens is converging or diverging. The distance from the centre of the lens to the focal point is called the focal length. A strongly curved lens usually bends light more and has a shorter focal length than a gently curved lens.

A convex lens is thicker in the middle than at the edges. It bends parallel rays inward so they meet at a focal point. If a distant object sends nearly parallel rays into the lens, a real image can form on the other side. A screen or camera sensor can capture that image.

A concave lens is thinner in the middle. It spreads parallel rays outward. The rays appear to come from a focal point on the same side as the object, producing a virtual image that cannot be projected onto a screen by that lens alone.

The object distance matters. With a convex lens, an object beyond the focal length can produce an inverted real image. An object placed closer than the focal length produces an upright, magnified virtual image. This is how a simple magnifying glass works. The same piece of glass can therefore behave differently because you moved the object. Cameras and eyes adjust image formation in different ways. A camera can move lens elements, while the human eye changes the shape of its flexible lens to focus objects at different distances.

You can predict an image by tracing a few principal rays. Real lenses can have imperfections, but the simple model explains the main behaviour. Where the rays meet tells you the image position. Its orientation and size can then be read from the diagram.

• •Draw the main axis and mark the focal points.
• •Send one ray parallel to the axis.
• •Refract that ray through or away from the focal point.
• •Draw a ray through the optical centre.
• •Find where the rays or their backward extensions meet.

Lenses form images through refraction at curved surfaces. Convex lenses bring rays together, while concave lenses spread them apart. Trace a few important rays and check the object distance, and you can predict where an image forms and what it will look like.