Convex & Concave Lenses (AQA GCSE Physics)
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Convex & Concave Lenses
A lens is a piece of equipment that forms an image by refracting light
There are two types of lens:
Convex
Concave
Convex Lenses
In a convex 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 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
The focal length is the distance from the lens to the principal focus
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
Parallel rays from a concave 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
Examiner Tips and Tricks
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
Students often find the lens diagrams quite tricky to draw. They do look complicated, but they are very similar to the reflection and refraction ray diagrams you have done before. Take your time when completing the ray diagrams, do them step by step. Keep practising until you feel confident.
Scan your exam paper at the start of the exam to see if you have a lens diagram to draw in any questions. That way you can leave a bit of extra time to complete the question without rushing. Always use a ruler and a sharp pencil, but take a rubber into the exam with you just in case!
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