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IGCSE 0625 · Section 3.2.3 · Core & Extended

Thin converging and diverging lenses.

Lenses use refraction to bend light to a specific point. Mastering the three standard rays for a converging lens guarantees you marks on almost every Paper 4.

The Key Idea

A converging (convex) lens brings parallel rays together at the principal focus. A diverging (concave) lens spreads parallel rays apart as if they came from the principal focus.

SECTION 01

The Interactive Lens.

Use the simulator below to construct standard ray diagrams. A real image is formed by solid lines meeting. A virtual image is formed when the solid lines diverge, and your brain traces them backwards (dashed lines) to a fake intersection.

The Diagram Trap

In Paper 4, drawing ray diagrams incorrectly guarantees zero marks for the question. Examiners specifically look for three things:

  • A ruler. Freehand rays are penalized.
  • Single lines. Thick, sketchy, or double lines are penalized.
  • Arrows. You must draw direction arrows on the solid light rays, but never on the dotted virtual extension lines.
Practice this topic →
Original Cambridge-style questions.
Practice drawing the standard rays and classifying images.