Lenses

• This bit is about how light acts when it hits a lens. Be ready for lots of diagrams on the next few pages.
• Lenses


  Form images by refracting light and changing its direction
• Types of lenses


  • Convex
  • Concave
• Convex lens


  • Bulges outwards
  • Causes rays of light parallel to the axis to be brought together (converge) at the principal focus
• Concave lens


  • Caves inwards
  • Causes rays of light parallel to axis to spread out (diverge)
• Axis of a lens

  A line passing through the middle of the lens
• Convex lens


  Also called converging lens
• Concave lens


  Also called diverging lens
• Principal focus of a convex lens


  Where rays hitting the lens parallel to the axis all meet
• Principal focus of a concave lens


  The point where rays hitting the lens parallel to the axis appear to all come from
• Focal length


  The distance from the centre of the lens to the principal focus
• Refraction in a convex lens

  1. Incident ray parallel to axis refracts through lens and passes through principal focus

  2. Incident ray passing through principal focus refracts through lens and travels parallel to axis

  3. Incident ray passing through centre of lens carries on in same direction
• Refraction in a concave lens

  1. Incident ray parallel to axis refracts through lens and travels in line with principal focus

  2. Incident ray passing through lens towards principal focus refracts through lens and travels parallel to axis

  3. Incident ray passing through centre of lens carries on in same direction
• The great thing about these rules is that they allow you to draw ray diagrams without bending the rays as they go into the lens and as they leave the lens. You can draw the diagrams as if each ray only changes direction once in the middle of the lens.