Converging lenses: how an image forms
Trace three rays and see why the same piece of glass turns the world into a camera, a magnifier and the human eye.
A converging lens bends light rays so they meet at a single point. This deceptively simple fact is enough to understand a camera, a microscope, a telescope and our own eye. Below, set the object distance and the focal length — the drawing recomputes the rays live.
Two rays are enough
A ray parallel to the optical axis heads for the focus on the far side after passing through the lens. A ray through the very centre keeps its direction. These two rules locate the image without any formula — it is where the rays cross again.
When the object is far away, the image forms just beyond the focus and is small — that is a camera. The closer the object moves to the focus, the farther and larger the image grows. Past the focus the image turns virtual and magnified — that is a magnifier.
A simplificationWe ignore aberrations, lens thickness and the colours of light. The principle of image construction stays exactly the same.
Bibliography (sample)
- 1 E. Hecht — "Optics", 5th ed., Pearson (2017). ISBN 978-0133977226
- 2 OpenStax — "University Physics, Vol. 3: Geometric Optics" (open access). openstax.org
- 3 R. P. Feynman — "The Feynman Lectures on Physics", Vol. I, ch. 27. caltech.edu
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