Ray Diagrams (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Features of ray diagrams

• Ray diagrams can be described using the following terms:

  • Principal axis

  • Principal focus, or focal point

  • Focal length

• The principal axis is defined as:

  A line which passes through the centre of a lens

• The principal focus, or focal point, is defined as:

  The point at which rays of light travelling parallel to the principal axis intersect the principal axis and converge or the point at which diverging rays appear to proceed

• Focal length is defined as:

  The distance between the centre of the lens and the principal focus

Converging lenses

• A thin converging lens brings parallel rays of light to a focus at a point 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

Converging lens diagram

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