AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to construct ray diagrams to illustrate the…
Full specification wording
Lenses (physics only): Students should be able to construct ray diagrams to illustrate the similarities and differences between convex and concave lenses.
Both lens types use the same two construction rays, one through the centre that carries straight on and one arriving parallel to the principal axis, and in both cases the image is found where the…
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 7 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Both lens types use the same two construction rays, one through the centre that carries straight on and one arriving parallel to the principal axis, and in both cases the image is found where the outgoing rays or their backward projections meet. The difference is what happens to the parallel ray. Through a convex lens it is refracted to pass through the principal focus on the far side, so the rays converge and a real image becomes possible. Through a concave lens it is refracted away from the axis so that it appears to come from the principal focus on the near side, so the rays diverge and only a virtual image is possible. A convex lens can give an inverted or an upright image depending on the object distance; a concave lens always gives an upright, diminished one.
What earns the marks
- The undeviated ray through the centre drawn correctly for both lenses.
- The parallel ray refracted through the far principal focus for the convex lens.
- The parallel ray refracted as if from the near principal focus for the concave lens.
- A stated similarity and a stated difference, each supported by the two diagrams.
Common mistakes and how to put them right
Drawing the same path for the parallel ray through both lenses.
Listing similarities and differences without drawing the diagrams the question asked for.
Using the convex lens symbol for both diagrams.
Giving a difference that is only about the shape of the glass, with no effect on the rays.
Exam tips
- Draw the two diagrams side by side with the object at the same distance, so the comparison is fair.
- Write the comparison in matched pairs: in the convex lens the rays converge, whereas in the concave lens they diverge.
- Use the correct symbol for each lens, since the symbols themselves carry marks.
Try the lesson
The start of the Lenses (physics only): Students should be able to construct ray diagrams to illustrate the similarities and differences between convex and concave lenses. lesson, made from checked teaching notes for this specification point.
Have a go first
In a ray diagram for a convex lens, which direction does a ray parallel to the principal axis travel after passing through the lens?
Listen
Both lens types use the same two construction rays, one through the centre that carries straight on and one arriving parallel to the principal axis, and in both cases the image is found where the outgoing rays or their backward projections meet. The difference is what happens to the parallel ray. Through a convex lens it is refracted to pass through the principal focus on the far side, so the rays converge and a real image becomes possible. Through a concave lens it is refracted away from the axis so that it appears to come from the principal focus on the near side, so the rays diverge and only a virtual image is possible. A convex lens can give an inverted or an upright image depending on the object distance; a concave lens always gives an upright, diminished one. A strong answer earns marks for these points. the undeviated ray through the centre drawn correctly for both lenses. the parallel ray refracted through the far principal focus for the convex lens. the parallel ray refracted as if from the near principal focus for the concave lens. a stated similarity and a stated difference, each supported by the two diagrams. Exam tip: Draw the two diagrams side by side with the object at the same distance, so the comparison is fair.
See it
Key words
Say what each word means before you turn the card.
shared construction rays
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converging versus diverging
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real versus virtual
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matched comparison
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See every definition
- shared construction rays
- Both lens types use the same two construction rays: one through the centre that carries straight on and one arriving parallel to the principal axis.
- converging versus diverging
- Through a convex lens the rays converge, whereas through a concave lens they diverge.
- real versus virtual
- A convex lens can form a real image, whereas a concave lens can only form a virtual image.
- matched comparison
- The comparison is written in matched pairs, such as in the convex lens the rays converge, whereas in the concave lens they diverge.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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