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Practice questions · Lenses

Three rays, drawn with a ruler.

Seven original Cambridge-style questions. They cover ray diagrams for real and virtual images, the difference between focal length and image distance, and the magnifying glass.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
The ray-diagram checklist

Two rays locate the image. A ruler earns the marks.

01
[2 marks]

Define the terms principal focus and focal length for a converging lens.

  • Principal focus: the point on the axis where rays travelling parallel to the principal axis converge after passing through the lens. ✓
  • Focal length: the distance from the centre of the lens to the principal focus. ✓
02
Analysis
[2 marks]

On a ray diagram, a student is asked to mark the focal length. They label the distance from the lens to the point where the image forms. Explain why this is wrong, and describe what they should have marked.

  • That distance is the image distance, which changes when the object moves. ✓
  • The focal length is the fixed distance from the lens to the principal focus F, the point where parallel rays are focused. ✓

Focal length is a fixed property of the lens. Image distance depends on where the object is.

03
Diagram
[4 marks]

The diagram shows a converging lens with its principal foci F and the points 2F marked. An object stands beyond 2F.

F F 2F 2F objectdraw the rays
Object beyond 2F in front of a converging lens.
(a) Draw two rays from the top of the object to locate the image. [2] (b) Describe the image fully (real or virtual, upright or inverted, magnified or diminished). [2]
F F 2F 2F object image
Rays converge between F and 2F: a real, inverted, diminished image.

(a) Parallel ray bends through F, central ray goes straight through the lens centre. They cross between F and 2F. ✓✓

(b) The image is real, inverted, and diminished. ✓✓

Object beyond 2F always gives a real, inverted, diminished image between F and 2F. This is the camera case.

04
[3 marks]

State the three standard rays used to locate the image formed by a converging lens.

  • A ray parallel to the axis, which then passes through the principal focus F on the far side. ✓
  • A ray through the centre of the lens, which continues straight and undeviated. ✓
  • A ray through the principal focus F in front of the lens, which then travels parallel to the axis. ✓
05
Analysis
[3 marks]

An object is placed between F and 2F in front of a converging lens. Describe the image formed, giving its nature, orientation, and size, and state one device that uses a lens in this way.

  • Real and inverted. ✓
  • Magnified (larger than the object), forming beyond 2F. ✓
  • Used in a projector (or a photographic enlarger). ✓

Between F and 2F: real, inverted, magnified. Beyond 2F: real, inverted, diminished. Worth memorising both.

06
Analysis
[3 marks]

A converging lens is used as a magnifying glass to read small print.

(a) Where must the print be placed relative to the principal focus? [1] (b) Describe the image seen (nature, orientation, size). [2]

(a) Closer to the lens than the principal focus (inside F). ✓

(b) The image is virtual, upright, and magnified. ✓✓

Inside F the rays diverge, so the image is virtual and on the same side as the object. This is the only converging-lens case giving an upright image.

07
Analysis
[2 marks]

A student draws a ray diagram in which the ray passing through the centre of the lens is shown bending slightly, and all of the rays are sketched freehand as slightly wavy lines. State the two drawing faults that would cost marks, and how to correct them.

  • The central ray must pass straight through the centre, undeviated. It should not bend. ✓
  • Rays must be single straight lines drawn with a ruler, not freehand wavy lines. ✓

Examiners reward clean construction: ruled single lines, arrows on each ray, undeviated central ray.