Lenses (Edexcel GCSE Physics)

• A lens is a piece of equipment that forms an image by refracting light
• There are two types of lens:
  • Converging
  • Diverging

Converging Lenses

• In a converging lens, parallel rays of light are brought to a focus
  • This point is called the principal focus

• This lens is sometimes referred to as a convex lens
• The distance from the lens to the principal focus is called the focal length
  • This depends on how curved the lens is
  • The more curved the lens, the shorter the focal length

The focal length is the distance from the lens to the principal focus

Diverging Lenses

• In a diverging lens, parallel rays of light are made to diverge (spread out) from a point
  • This lens is sometimes referred to as a concave lens

• The principal focus is now the point from which the rays appear to diverge from

Parallel rays from a diverging lens appear to come from the principal focus

• The power of a lens is related to:
  • The focal length of the lens
  • The shape of the lens

• The power of a lens is a measure of how strongly it focuses the light
  • The more curved the lens, the shorter the focal length
  • The shorter the focal length, the greater the power of the lens

The power of a lens depends on its focal length

• Power is defined by the equation:

• Power is measured in dioptres
• Sometimes the focal length is negative
  • A negative focal length is often used for a diverging lens