CoachSend a bundle of parallel rays — sunlight serves — through a converging lens: the bent rays all cross at one point behind it, the focal point. The distance from lens to focal point is the focal length : the lens’s one defining number. Strongly curved, fat lenses bend hard — short ; gently curved ones bend little — long .
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Example 60.8 (Inside the focal length)
Hold a converging lens closer to a stamp than its focal length, and the picture changes character: no image lands on any screen — instead, looking through the lens, you see the stamp upright, in place, and enlarged. The lens bends the stamp’s rays so they reach the eye as if diffused by a grander stamp behind the glass — a made-to-order illusion, cousin to the mirror’s phantom world. Watchmakers, stamp collectors and your own past self with the childhood magnifying glass have all used it; the full ray-geometry of this trick is a jewel of the High School volume.