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Light & Colour · Question 07 of 20

The magnifying glass

A curved piece of glass tells your eye a useful lie: that tiny things are sitting much closer, and much larger, than they are.

Plate X — The thin lens 1/dᵢ = 1/f − 1/dₒ
Slide the object through the focal point and watch the image flip.
F Fobjectimageoutside F: rays really cross — you could catch this on paper
FIG. X — THE THIN LENS
Object distance 40 cm · outside F
Slide the object through the focal point and watch the image flip.
Focal length 22 cm
Image
real · inverted
Magnification
1.2×
Bring the object close to the lens (inside the F dot) and the rays leave still spreading apart — your eye traces them back to a bigger, upright ghost. That's a magnifying glass. Push it past F and the rays truly cross: a real, upside-down image, like a cinema projector.
The short answer

A curved lens bends the light rays so they spread apart by the time they reach your eye, tricking it into seeing the object much bigger.

What's actually happening

A magnifying glass is refraction with a plan. Curve both faces of a glass disc outward and every ray that passes through gets bent toward the middle — rays through the fat centre barely at all, rays near the thin edge quite sharply. Parallel light entering this lens converges to a single spot: the focal point. The distance to that spot, the focal length, is the lens's one defining number.

The magnifying trick happens when you hold the object closer to the lens than that focal length. The lens bends the object's rays toward each other, but they are diverging too steeply to actually meet — they leave the lens still spreading, just less so. Your eye catches these gently-spreading rays and runs its usual straight-line reconstruction: it traces them back to a much bigger object, sitting further away. That enlarged phantom is a virtual image — nothing is actually there, which is why you cannot project it onto paper.

Hold the lens far from the page instead, beyond the focal length, and the rays really do cross. Now you get a real image — one that can land on a surface. Sunlight focused to a scorching dot is the most famous real image in childhood; a cinema projector and your own retina work on exactly the same principle.

Try it at home A water-drop microscope
  1. 1Lay a sheet of clear plastic (or cling film pulled taut) over a printed page.
  2. 2Place a single small water drop on the plastic and look straight down through it — the letters beneath bulge larger.
  3. 3Add water to fatten the drop and the magnification grows: a fatter drop is a more sharply curved, shorter-focus lens.