Lenses and Ray Diagrams
A converging lens bends light to a point. Once you can draw the two standard rays, every question becomes a matter of reading the diagram — and describing the image in the three words the mark scheme wants.
1. The two kinds of lens
Converging (convex) — thicker in the middle; brings parallel rays together at the principal focus. Diverging (concave) — thinner in the middle; spreads parallel rays apart.
Convex converges; concave caves in. Confusing the two was a recorded error, and it changes every ray in the diagram. At 0625 the great majority of questions use the converging lens.
Key terms:
- Principal axis — the horizontal line through the centre of the lens
- Optical centre (C) — the middle of the lens; a ray through it passes straight on, undeviated
- Principal focus (F) — where rays parallel to the axis converge
- Focal length (f) — distance from the lens to F
A lens has a focus on both sides, at equal distances. 2F is twice the focal length from the lens, and it matters constantly in these questions.
A fatter (more curved) lens has a SHORTER focal length and bends light more strongly.
2. Drawing a ray diagram
Two rays are enough to locate the image; a third is a useful check.
Ray 1: parallel to the principal axis → refracts through F (on the far side). Ray 2: through the optical centre → continues straight, undeviated. Ray 3 (check): through F on the near side → emerges parallel to the axis.
The image forms where the rays cross.
Method:
- Draw the principal axis and the lens (a vertical line with outward arrows for converging)
- Mark F and 2F on both sides
- Draw the object as an upright arrow on the axis
- Draw at least two rays from the top of the object
- Mark the image where they meet, and draw it as an arrow
Use at least two rays — tutors flagged this specifically, and one ray cannot fix a position.
Ray 1 must pass exactly through F, not near it. Sloppiness here shifts the image and loses the accuracy mark.
Use a ruler and a sharp pencil, and put arrows on your rays to show the direction of travel.
Mark the lens position clearly — forgetting to was a recorded error.
3. The nature of the image — the table that answers most questions
For a converging lens, everything depends on where the object is relative to F and 2F:
| Object position | Image position | Nature of image |
|---|---|---|
| Beyond 2F | between F and 2F | real, inverted, diminished |
| At 2F | at 2F | real, inverted, same size |
| Between F and 2F | beyond 2F | real, inverted, magnified |
| At F | — | no image (rays emerge parallel) |
| Inside F | same side as object | virtual, upright, magnified |
Describe the image with THREE things: real or virtual, upright or inverted, magnified, diminished or same size. Mark schemes award each, and questions often ask you to “circle three items describing the nature of the image”.
The image is only upright when the object is INSIDE the focal length. Every real image from a converging lens is inverted.
An object at F produces no image, because the emerging rays are parallel and never meet. A recorded error was thinking no image forms when the object is at or beyond F — beyond F a real image certainly does form.
Real vs virtual:
A real image is formed where light rays actually meet. It can be projected onto a screen and is always inverted (with a single converging lens). A virtual image is formed where rays only appear to come from — traced backwards as dashed lines. It cannot be projected and is upright.
Draw virtual rays as DASHED lines. The light does not really travel along them.
Uses: camera and projector (object beyond F, real image); magnifying glass (object inside F, virtual magnified image); the eye (real inverted image on the retina).
4. Magnification
magnification = image height ÷ object height magnification = image distance ÷ object distance
Magnification has no units — it is a ratio.
- m > 1 → image is magnified
- m < 1 → image is diminished
- m = 1 → same size
Example: an object 2 cm tall gives an image 6 cm tall → m = 6/2 = 3.
Measure heights carefully from your diagram and give the answer to a sensible accuracy. Tutors warned that a hand-drawn diagram gives an approximate value — in the exam you are usually given actual measurements to use instead.
Scope note. Some lessons used the lens formula 1/f = 1/u + 1/v and its sign conventions. This is not on the 0625 syllabus — 0625 requires ray diagrams and the magnification ratio, not the lens equation. It belongs to A-level. Check your own syllabus document before revising it.
5. Finding the focal length experimentally
The distant object method:
- Point the lens at a distant object (rays arriving are effectively parallel)
- Move a screen until the image is sharp
- Measure the distance from lens to screen — this is the focal length
Adjust patiently until the image is genuinely sharp. Tutors made this point about practical work; a blurred image gives a poor focal length.
6. Mistakes that cost marks
Confusing converging with diverging lenses.
Drawing a ray through F inaccurately.
Using only one ray.
Forgetting to mark F and 2F on both sides.
Not putting arrows on the rays.
Drawing virtual rays as solid lines.
Giving fewer than three descriptors for the image.
Saying a real image is upright.
Thinking no image forms beyond F.
Confusing image distance with object distance in magnification.
Giving units for magnification.
Frequently asked questions
What is a converging lens? A convex lens, thicker in the middle, which brings parallel rays to the principal focus.
What is the focal length? The distance from the lens to the principal focus.
Which rays do I draw? One parallel to the axis (refracting through F) and one through the optical centre (undeviated).
What is a real image? One formed where rays actually meet — it can be shown on a screen and is inverted.
What is a virtual image? One formed where rays only appear to come from — it cannot be projected and is upright.
When is the image magnified and upright? When the object is inside the focal length — the magnifying glass case.
What happens if the object is at F? No image forms; the rays emerge parallel.
How do I calculate magnification? Image height ÷ object height (or image distance ÷ object distance). It has no units.
How do I find the focal length in the lab? Focus a distant object onto a screen and measure the lens-to-screen distance.
Do I need the lens formula? Not for 0625 — that is A-level material.
Quick revision checklist
- I can tell converging from diverging lenses
- I know the principal axis, optical centre, F, 2F and focal length
- I can draw the parallel ray and the centre ray
- I use at least two rays and a ruler
- I mark F and 2F on both sides
- I put arrows on rays and dash the virtual ones
- I can state the image nature for every object position
- I describe images with three properties
- I know real images are inverted and virtual ones upright
- I know no image forms with the object at F
- I can calculate magnification and know it has no units
- I can describe the distant-object method for focal length
- I know the lens formula is beyond 0625
These notes cover lenses and ray diagrams in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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