Lenses (physics only) — AQA GCSE Physics
Test yourself on Lenses (physics only) with AQA GCSE practice questions.
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Lenses (physics only) explained
Send a set of rays parallel to the principal axis into a convex lens and every one of them is refracted so that they all cross at a single point on the axis beyond the lens.
Read the full explanation
That point is the principal focus, and because real light rays actually meet there it is a real focus: place a screen at that point and you see a bright spot. Rays that arrive parallel to each other but at an angle to the axis also converge to a point, but one that lies off the axis. A fatter, more strongly curved lens refracts the rays more, so they meet closer to the lens. Sunlight focused in this way concentrates the energy into a small area, which is why the spot can scorch paper.
Your focus
- Draw the paths of three rays parallel to the principal axis passing through a convex lens.
- Label the principal focus and the principal axis on a lens diagram.
- Explain why a more strongly curved convex lens has a shorter focal length.
Lenses (physics only) exam tips
Marking Points
- one mark for stating that rays parallel to the principal axis converge after passing through the lens
- one mark for naming the point where they cross as the principal focus
- one mark for saying that the rays actually meet there, so the focus is real
- one mark for linking a more strongly curved lens to a focus closer to the lens
Examiner Tips
- 💡Draw the principal axis first and mark F on both sides at equal distances from the lens.
- 💡Rays parallel to the axis pass through F; that is the first of the standard construction rays.
- 💡Label the principal focus F on the diagram, because the label itself can be the mark.
Common Mistakes
- drawing the rays crossing at the centre of the lens instead of at the principal focus
- assuming a lens has only one principal focus, when there is one on each side
- drawing the incoming rays already converging before they reach the lens
- saying the principal focus is where the object must be placed