Lens Lab

A magnifying glass, a camera and a film projector are the same lump of curved glass — the only thing that changes is how far away you hold it. Drag the arrow along the bench and watch the picture the lens makes grow, flip over, and at one exact spot vanish completely.

drag the arrow · drag the screen · or use arrow keys

What the lens is doing

—object distance
—image distance
—magnification
—image height
— — —

Same lens, five spots on the bench

Knobs

7.0 cm
How strongly the glass bends light. Small number = fat, strong lens.
10.5 cm
How far the arrow sits in front of the lens. Same as dragging it.
4.0 cm
How tall the arrow is. The lens opening grows to match it.

The screen

Reading the minus sign. Distances measured out past the lens count as positive. When the image distance comes out negative the picture is sitting back on the same side as the object — light never actually goes there, so no screen in the world can catch it. That is the whole difference between real and virtual.