Thin Lenses (Cambridge O Level Physics)

Revision Note

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Dan MG

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Features of Lens Diagrams

  • 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

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

Converging Lens

Lens focus, IGCSE & GCSE Physics revision notes

The focal length is the distance from the lens to the principal focus. In a converging lens, parallel light rays converge on 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

Diverging Lens, downloadable IGCSE & GCSE Physics revision notes

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

Representing Lenses

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

Symbols for Lenses

convex-concave-symbols, IGCSE & GCSE Physics revision notes

Concave and convex symbols

Examiner Tip

Make sure you remember the symbol for each type of lens, as you will be expected to draw these for ray diagrams in your exam!

To remember which lens is converging or diverging, think of the following: Convex lens = Converging

If you struggle with the difference between convex and concave, remember that a concave shape curves inwards, like a cave.

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Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.