Concave Lenses and Magnification

• Concave lens


  A lens that is thinner at the centre than at the edges, causing light rays to diverge
• Drawing a ray diagram for an image through a concave lens


  1. Pick a point on the top of the object

  2. Draw a ray going from the object to the lens parallel to the axis of the lens

  3. Draw another ray from the top of the object going right through the middle of the lens

  4. The incident ray that's parallel to the axis is refracted so it appears to have come from the principal focus. Draw a ray from the principal focus

  5. The ray passing through the middle of the lens doesn't bend

  6. Mark where the refracted rays meet. That's the top of the image

  7. Repeat the process for a point on the bottom of the object
• Image produced by a concave lens

  • Virtual
  • Upright
  • Smaller than the object
  • On the same side of the lens as the object
• Magnifying glass


  A convex lens used to create a magnified virtual image
• How a magnifying glass works

  1. The object being magnified must be closer to the lens than the focal length

  2. The light rays don't actually come from the place where the image appears to be

  3. You can't project a virtual image onto a screen
• Magnification formula


  magnification = image height / object height
• Calculating magnification


  • A coin with diameter 14 mm is placed behind a magnifying lens. The virtual image produced has a diameter of 35 mm. The magnification of the lens at this distance is 35/14 = 2.5
• Magnification is a ratio, so it doesn't have any units
• You can also find the magnification by dividing the distance between the image and the lens by the distance between the object and the lens