Ray Diagrams (Cambridge International IGCSE Physics)

Features of Ray Diagrams

Spec point — 3.2.6 · Define and use the terms focal length, principal axis and principal focus (focal point)

• 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 & diverging lenses

Spec point — 3.2.6 · Describe the action of thin converging and thin diverging lenses on a parallel beam of light

• 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

Converging lens diagram

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

Diverging lens diagram

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

Representing lenses

• In diagrams, the following symbols are often used to represent each type of lens:

Converging and diverging lens symbols

Concave and convex lens symbols

Exam Hint To remember which lens is converging or diverging, think of the following: Convex lens = Converging. You need to be able to describe how the lenses affect the light rays.