Lenses (Oxford AQA IGCSE Physics)

Revision Note

Convex Lenses

  • A lens is a piece of equipment that forms an image by refracting light

  • There are two types of lenses:

    • Convex

    • Concave

  • In a convex lens, parallel rays of light are brought to a focus

    • This point is called the principal focus (sometimes referred to as the focal point)

  • This lens is sometimes referred to as a converging 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

Light rays passing through a converging lens

A convex lens with three parallel rays incident on the lens that are refracted towards the principal focus at a distance equal to the focal length away from the lens, for GCSE & IGCSE Physics revision notes
When light rays pass through a convex lens they converge at the principal focus on the other side

Concave Lenses

  • In a concave lens, parallel rays of light are made to diverge (spread out) from a point

    • This lens is sometimes referred to as a diverging lens

  • The principal focus is now the point from which the rays appear to diverge from

Light rays passing through a diverging lens

A diagram that shows an image formed by a diverging lens. Incident parallel rays are spread out after passing through the lens. The principal focus where the rays meet is at a distance equal to the focal length from the lens, for GCSE & IGCSE Physics revision notes
When parallel light rays pass through a diverging lens they diverge from the principal focus, through the lens and onto the other side

Representing Lenses

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

Representing convex and concave lenses

The diagram shows a convex lens and it's lens diagram symbol on the left. This is a vertically aligned double headed arrow. The concave lens on the right is shown with its lens diagram symbol. This is a vertically aligned line with reverse arrows on either end.
Convex and concave lenses should be represented differently in lens diagrams

Examiner Tip

Remember the symbol for each lens type, 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

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