Ray Diagrams (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• 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

• 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