Lenses (Edexcel GCSE Physics): Revision Note

Converging & Diverging Lenses

• 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

Lens Power

• 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:

$power=\frac{1}{focallength}$

• Power is measured in dioptres

• Sometimes the focal length is negative

  • A negative focal length is often used for a diverging lens