Thin Lenses (CIE IGCSE Physics)

• Lens 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 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

Representing Lenses

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

Concave and convex symbols