IGCSE §3: Waves · IGCSE
Lenses and Images
Revision notes on Lenses and Images for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Converging (convex) lens — thicker in the middle than at the edges. Rays arriving parallel to the axis are brought together at the principal focus.
Diverging (concave) lens — thinner in the middle than at the edges. Parallel rays are spread out, appearing to come from a principal focus on the near side.
Principal focus (F) — the point at which rays parallel to the principal axis converge, or from which they appear to diverge.
Focal length (f) — the distance from the centre of the lens to the principal focus. A fatter, more strongly curved lens has a shorter focal length and bends light more.
Real image — formed where light rays actually meet. It can be projected onto a screen. A cinema screen and the image on a camera sensor are real.
Virtual image — formed where rays only appear to come from, after being traced backwards. It cannot be caught on a screen. The image in a plane mirror and the one seen through a magnifying glass are virtual.
Drawing a ray diagram. Two rays from the top of the object are enough, and these are the two easiest:
- A ray parallel to the principal axis, which refracts through the principal focus.
- A ray through the centre of the lens, which passes straight on undeviated.
Where they cross is the top of the image. If they diverge, trace them back to find the virtual image.
What a converging lens does depends on where the object is:
- Beyond 2F — a real, inverted, diminished image between F and 2F. This is a camera.
- Between F and 2F — a real, inverted, magnified image beyond 2F. This is a projector.
- Closer than F — a virtual, upright, magnified image on the same side as the object. This is a magnifying glass.
A diverging lens always gives the same answer: a virtual, upright, diminished image, wherever the object is.
Magnification:
magnification = image height / object height
It has no unit, being a ratio of two lengths. A magnification greater than 1 means the image is larger than the object.
Example: a magnifying glass only magnifies while the object is nearer than the focal length. Move it further away and the image flips over and shrinks — which is why a lens held at arm's length shows the room upside down.
4 questions on Lenses and Images
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
Start Lenses and Images