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Lesson: Chapter 5 — Optics

5.4.1. Convex lenses 2 of 4

Focus and focal length

Rays parallel to the principal axis enter a convex lens, bend inwards — they converge — and meet at one point on the other side: the focus.

A parallel beam passing through a convex lens and converging to the focus F on the far side.
  • Entering the lens from the air, a ray goes from a rarer into a denser medium and bends towards the normal.
  • Leaving the lens it goes from denser to rarer and bends away from the normal.
  • Both times the ray bends towards the principal axis, and every ray parallel to the axis ends up passing through the same point.

Focus and focal length

That point is the focus (F) of the lens, and its distance from the optical centre is the focal length (f). Light can fall on a lens from either side, so a lens has two foci, one on each side, equally far from the optical centre.