Lenses are used in optical devices such as the camera. Although a digital camera is very different from the very first cameras made over 160 years ago, they both contain a lens that is used to form an image.
A lens works by changing the direction of light passing through it. The curved shape of the lens surfaces refracts the rays so they meet at a point.
Different lens shapes can be tested using this arrangement.
- A converging lens makes parallel rays to converge to a focus. The point where parallel rays are focused is the principal focus or focal point of the lens.
- A diverging lens makes parallel rays diverge. The point where the rays appear to come from is the principal focus of the lens.
In both cases, the distance form the centre of the lens to the principal focus is the focal length of the lens.
A real image is formed by a converging lens if the object is further away than its principal focus.
A virtual image is formed by a diverging lens and by a converging lens if the object is nearer than the principal focus.