Ray diagrams are one of the most reliable places to pick up marks in the Light section of O-Level Pure Physics (6091). The reason is simple: the marker is looking for a small, fixed list of features. If you know that list, you can score full marks even under time pressure. Below is exactly how we drew them when we sat the paper, and the small habits that separate a 3/3 from a 1/3.
The vocabulary the mark scheme expects
Before drawing anything, be sure of these terms:
- Principal axis: the horizontal line through the centre of the lens, perpendicular to it.
- Optical centre (C): the middle of the lens, on the principal axis.
- Principal focus (F): the point where rays parallel to the principal axis converge after refraction. A converging lens has one on each side.
- Focal length (f): distance from the optical centre to F.
- Real image: formed where light rays actually meet. Can be caught on a screen.
- Virtual image: formed where rays only appear to come from. Cannot be caught on a screen.
When describing an image you must give three properties: real or virtual, upright or inverted, and magnified, diminished or same size. Many students give two and lose the mark.
The three standard rays
You only ever need two rays to locate an image, but drawing a third is a useful check and often earns credit.
- Parallel ray: a ray from the top of the object travelling parallel to the principal axis refracts at the lens and passes through F on the far side.
- Central ray: a ray from the top of the object through the optical centre C continues in a straight line, undeviated.
- Focal ray: a ray from the top of the object passing through the near F emerges parallel to the principal axis.
The image of the object's tip is where these rays intersect. Because the object sits on the principal axis, the base of the image sits on the axis too, so you just drop a straight line down.
Step-by-step method
- Draw the principal axis with a ruler, then the lens as a vertical line (or a thin lens shape) crossing it at C.
- Mark F and 2F on both sides, evenly spaced. Neat, symmetrical spacing matters; markers do check.
- Draw the object as a vertical arrow standing on the principal axis, pointing up.
- Draw rays 1 and 2 from the tip of the arrow. Refract them at the lens, not before or after.
- Extend the refracted rays until they cross. Put arrowheads on every ray to show the direction of travel.
- Draw the image as an arrow from the axis to the intersection point, then label it and state its properties.
If you are told the focal length and object distance, use graph paper and a stated scale, for example 1 cm to 5 cm. Scale drawings can be asked to find image distance and magnification.
The five object positions you must know
| Object position | Image position | Image properties |
|---|---|---|
| Beyond 2F | Between F and 2F | Real, inverted, diminished |
| At 2F | At 2F (other side) | Real, inverted, same size |
| Between F and 2F | Beyond 2F | Real, inverted, magnified |
| At F | At infinity | No image formed |
| Between F and lens | Same side as object | Virtual, upright, magnified |
Useful links to real devices: camera (object beyond 2F), projector (object between F and 2F), magnifying glass (object inside F).
The virtual image case
This is where most marks are dropped. When the object is closer to the lens than F, the refracted rays diverge and never meet on the far side. You must:
- Draw the refracted rays diverging away from the lens with solid lines and arrowheads.
- Extend them backwards with dashed lines to the point where they appear to meet.
- Draw the image on the same side as the object, upright, larger, and label it "virtual image".
Dashed construction lines carry no arrowheads. Solid line for real light, dashed for imagined extensions. That single habit is worth a mark.
Magnification and the numbers
Linear magnification is
m = image height / object height = v / u
where v is image distance and u is object distance from the lens. If your scale drawing gives an image 2.5 times taller, state m = 2.5 with no unit. Magnification has no units, so writing "2.5 cm" loses the mark.
Common mistakes to avoid
- Freehand lines. Always use a ruler and a sharp pencil.
- Bending rays before or after the lens instead of at it.
- Forgetting arrowheads on rays.
- Drawing rays from the base of the object rather than the tip.
- Unequal spacing between F and 2F.
- Describing the image with only two of the three properties.
- Using solid lines for virtual ray extensions.
Practise by drawing all five cases from memory in under ten minutes. Once the pattern is automatic, these questions become free marks in Paper 2.
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