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.
Most people think a magnifying glass works only because the curved glass is thicker in the middle. In fact what matters is that it bends rays inward; held inside its focal length, the rays leave still spreading and your eye traces them back to a bigger virtual image.
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.
A magnifying glass tells your eye a useful lie: held closer than its focal length, it makes rays spread gently so you trace them back to a larger image that is not really there.
- 1Lay a sheet of clear plastic (or cling film pulled taut) over a printed page.
- 2Place a single small water drop on the plastic and look straight down through it: the letters beneath bulge larger.
- 3Add water to fatten the drop and the magnification grows: a fatter drop is a more sharply curved, shorter-focus lens.
Common questions
When the object is inside the focal length, the lens forms a virtual image. Nothing is actually there for light to land on, so it cannot be cast onto a surface, unlike a real image.
Held far from the page, beyond its focal length, the lens makes the rays truly cross and form a real image. Focused sunlight concentrates into a scorching dot that can be projected.
It is the strength in dioptres, which is one divided by the focal length in metres. A bigger number means a stronger bend and a shorter focus.