What mistakes do students make with Lenses (physics only)?

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.

What are the exam tips for Lenses (physics only) in Physics AQA GCSE?

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.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

In a convex lens, parallel rays of light are brought to a focus at the…

Full specification wording

Lenses (physics only): In a convex lens, parallel rays of light are brought to a focus at the principal focus.

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.

Checked against the AQA 8463 specification (opens in a new tab) on 22 September 2026.

Point 2 of 9
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Stating that rays parallel to the principal axis converge after passing through the lens.
Naming the point where they cross as the principal focus.
Saying that the rays actually meet there, so the focus is real.
Linking a more strongly curved lens to a focus closer to the lens.

Common mistakes and how to put them right

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.

Exam 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.

Try the lesson

The start of the Lenses (physics only): In a convex lens, parallel rays of light are brought to a focus at the principal focus. lesson, made from checked teaching notes for this specification point.

Have a go first

What happens to parallel rays of light when they pass through a convex lens?

Tip: press 1 to 4 to choose

Listen

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. 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. A strong answer earns marks for these points. stating that rays parallel to the principal axis converge after passing through the lens. naming the point where they cross as the principal focus. saying that the rays actually meet there, so the focus is real. linking a more strongly curved lens to a focus closer to the lens. Exam tip: Draw the principal axis first and mark F on both sides at equal distances from the lens.

See it

A top-down flowchart. A box labelled 'Parallel rays of light' leads to 'Convex lens', which leads to 'Rays refracted', then to 'Rays cross at a single point on the axis', then to 'Principal focus', then to 'Rays actually meet there, so the focus is real'. Separately, a box labelled 'More strongly curved lens' leads to 'Refracts the rays more', which leads to 'Rays meet closer to the lens'. — It shows parallel rays being refracted by a convex lens to cross at the principal focus, and that a more strongly curved lens brings the focus closer to the lens.

Key words

Say what each word means before you turn the card.

principal focus

Try answering aloud or in your head before turning the card.

real focus

Try answering aloud or in your head before turning the card.

See every definition

principal focus: The point on the axis beyond the lens where parallel rays of light are brought to a focus.
real focus: The focus is real because real light rays actually meet there, so a screen placed at that point shows a bright spot.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

$The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A box labelled 'Parallel rays of light' leads to 'Convex lens', which leads to 'Rays refracted', then to 'Rays cross at a single point on the axis', then to 'Principal focus', then to 'Rays actually meet there, so the focus is real'. Separately, a box labelled 'More strongly curved lens' leads to 'Refracts the rays more', which leads to 'Rays meet closer to the lens'.$

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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