Waves: Concave Lenses and Magnification

Waves: Concave Lenses and Magnification

Properties of Concave Lenses

  • A concave lens is thinner at the centre than at the edges.
  • They are also known as diverging lenses as they spread out light rays that are travelling parallel to their axis.
  • The line through the centre of the lens is called the principal axis. Light rays that are parallel to this line get diverted outwards after passing through the lens.
  • The point where rays parallel to the principal axis appear to come from after passing through or reflecting off a lens is known as the principal focus. For a concave lens, this is on the same side of the lens as the incoming light.
  • A concave lens will always produce a virtual, upright image, smaller than the object.

Diagramming Concave Lenses

  • When ray tracing for a concave lens, three rays are most commonly used - one parallel to the principal axis, one through the centre of the lens, and one towards the principal focus.
  • The ray parallel to the principal axis will be diverted so it appears to come from the principal focus.
  • The ray through the centre of the lens will carry on straight through without any deviation.
  • The ray travelling towards the principal focus on the other side of the lens will be refracted so it is now travelling parallel to the principal axis.
  • The image is then found where these refracted rays cross.

Uses of Concave Lenses

  • Concave lenses are used in a variety of optical instruments.
  • They find use in spectacle lenses for the correction of myopia or short-sightedness.
  • In telescopes and cameras, they can be used in combination with convex lenses to control the focussing and manipulation of light.

Magnification and Lenses

  • The magnification of a lens is the ratio of the height of the image to the height of the object.
  • A concave lens always gives a magnification of less than 1, meaning the image is always smaller than the object.
  • The magnification can be calculated using the formula: magnification = image height / object height.

Practicals with Concave Lenses

  • Verification of properties of a concave lens, such as formation of a diminished image, can be investigated in the lab using ray boxes and lenses.
  • Experimentation is also possible by projecting an image onto a screen and measuring image and object heights to calculate magnification.
  • Object distance, image distance and the lens focal length relationship can be validated using the thin lens equation.