Waves: Lenses

Waves: Lenses

Types of Lenses

  • There are two types of lenses, convex (converging) and concave (diverging).
  • A convex lens is thicker in the middle than at the edges and converges light rays towards a point.
  • A concave lens is thinner in the middle than at the edges and spreads out, or diverges, light rays.

Convex Lens Behaviour

  • A convex lens causes parallel rays of light to converge at the focal point.
  • The distance from the centre of a lens to its focal point is called the focal length.
  • The middle of the lens is called the optical centre.

Concave Lens Behaviour

  • A concave lens causes parallel rays of light to appear as if they have come from one point, or a virtual focal point.
  • Even though the light rays do not actually meet, they appear to come from the focal point on the same side of the lens as the incoming light.

Ray Diagrams

  • In ray diagrams, incoming parallel rays are drawn along a line called the principal axis.
  • When constructing ray diagrams, one ray is drawn from the top of the object through the optical centre, while others are drawn parallel to the principal axis and then refracted through the focal point.

Lens Power and Focal Length

  • The power of a lens is the inverse of its focal length. Power is measured in dioptres (D) and focal length is measured in metres (m).
  • A lens with a short focal length has high power.
  • A lens with a long focal length has lower power.

Lens Equations

  • The formula relating image height, object height, and magnification is: magnification = image height / object height.
  • The formula relating focal length (f), object distance (u), and image distance (v) is: 1/f = 1/u + 1/v.

Uses of Lenses

  • Lenses are used in a wide range of optical devices, such as cameras, microscopes, telescopes and spectacles.
  • In these devices, lenses are often combined to form compound lenses to improve image quality or achieve other specific purposes, like zooming in cameras.