How do thin lenses focus or disperse light?

Thin lenses focus or disperse light by bending (refracting) the light rays as they pass through the lens.

A thin lens is a lens whose thickness allows light to be refracted, or bent, as it passes through. This refraction is due to the change in speed of light when it moves from one medium (air) into another (glass or plastic of the lens). The amount of bending depends on the shape of the lens and the material from which it is made.

There are two main types of thin lenses: converging and diverging. Converging lenses, also known as convex lenses, are thicker in the middle than at the edges. When parallel light rays enter a converging lens, they are refracted towards the centre and meet at a point known as the focal point. This is why converging lenses are often used in devices that require focusing light, such as cameras and microscopes.

On the other hand, diverging lenses, also known as concave lenses, are thinner in the middle than at the edges. When parallel light rays enter a diverging lens, they are refracted away from the centre and appear to come from a single point behind the lens. This is why diverging lenses are often used in devices that require dispersing light, such as in glasses for people with myopia (short-sightedness).

The power of a lens, which is a measure of how much it can bend light, is determined by its focal length. The shorter the focal length, the more powerful the lens. This is because a shorter focal length means the light rays are bent more sharply, allowing the lens to bring the light to a focus in a shorter distance.

In summary, thin lenses focus or disperse light by refracting the light rays as they pass through, with the amount of bending depending on the shape and material of the lens.