How mirrors and lenses form images by reflecting and bending light
Light reflects from a surface so that the angle of incidence equals the angle of reflection, with both rays and the normal in the same plane. A plane mirror forms an upright, virtual image the same size as the object. Concave mirrors can form real, inverted images and are used in torches and shaving mirrors, while convex mirrors always form small, upright images and give a wide view, which is why they are used as rear-view mirrors.
Refraction is the bending of light as it passes from one medium to another because its speed changes. Snell’s law says sin i / sin r is constant for a given pair of media, and this ratio is the refractive index. A convex lens converges light and a concave lens diverges it. For both mirrors and lenses, the mirror formula 1/v + 1/u = 1/f and lens formula 1/v − 1/u = 1/f link object distance, image distance and focal length. The power of a lens is P = 1/f in dioptres, with f in metres.
Why it matters
Mirrors and lenses are inside spectacles, cameras, microscopes, telescopes, car headlights and projectors. Refraction explains why a pool looks shallower than it is and why a pencil appears bent in water. Doctors use lens power to prescribe glasses. This chapter prepares you for the human eye and for ray optics in Class 12.
Key terms
- Refractive index
- The ratio of the speed of light in a vacuum to its speed in a medium.
- Focal length
- The distance between the optical centre of a lens or pole of a mirror and its principal focus.
- Virtual image
- An image that cannot be formed on a screen because light rays only appear to come from it.
- Power of a lens
- The reciprocal of focal length in metres, measured in dioptres.