Converging lenses — Physics, 14–17 years
A converging lens bends parallel light rays towards one another. Its focal length and the object’s distance determine where an image forms and whether it is larger, smaller, upright or inverted.
Bringing rays together
A converging lens is thicker in the middle than at the edges. Refraction at its two curved surfaces turns parallel rays towards a common region called the focus. The distance from the lens to that focus is the focal length, a key measure of how strongly the lens bends light.
Why predict the image?
An optical instrument is only useful if its image appears in the right place and with the right size. Early lens makers needed more than trial and error to build spectacles, telescopes and cameras. The lens equations turn the shape and distances into a prediction of the image.
Finding an image distance
A converging lens has focal length f = 10 cm, and an object is 30 cm away, so u = 30 cm. Use 1/f = 1/u + 1/v: 1/10 = 1/30 + 1/v. Thus 1/v = 1/10 − 1/30 = 1/15, giving v = 15 cm. The image is real and inverted, with magnification 15/30 = 0.5.
The image-direction trap
It is easy to assume that a lens always makes an upright, enlarged picture because that is what a magnifying glass can do. In fact, a converging lens gives different images at different object distances. Beyond the focal point it can form a real, inverted image; inside the focal length it forms a virtual, upright, enlarged one.
Lenses around us
A camera lens forms a real image on a sensor, where software or electronics records it. A projector uses a lens to make a small bright image appear large on a screen. Spectacles use carefully chosen lenses to move the image onto the retina; the same physics works even though the devices have different purposes.
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