Lenses and Images; the Eye and Optical Instruments
How a curved piece of glass bends light to build an image — and how your own eye does the same job
Big Idea: A lens is a transparent body bounded by curved surfaces that refracts light. A converging (convex) lens gathers parallel rays to a single focus; a diverging (concave) lens spreads them apart. By tracing just a few standard rays we can predict exactly where the image of an object appears, how large it is, and whether it is real or virtual. Your eye, a camera, a magnifying glass and a telescope are all the same physics applied in different ways.
By the end you will be able to:
- tell a converging lens from a diverging one and name the optical centre, principal axis, focus and focal length;
- use optical power D = 1/F and measure it in diopters;
- construct an image with the three principal rays;
- apply the thin-lens equation 1/F = 1/d + 1/f and the magnification Γ = H/h = f/d;
- explain how the eye focuses, why near- and far-sightedness happen, and which lens corrects each;
- describe how a magnifying glass, microscope, telescope, camera and projector work.