1What a lens does to light
A lens bends rays inward — until they cross
You only need two ideas. Watch each one move:
It bends rays inward
A lens is fatter in the middle. Straight rays go in, and the glass tilts every one of them inward — toward each other.
They all meet at one spot
Bent inward, the rays aim at one point and cross there. That crossing spot is the focus point.
2Two things a lens can do
Before the cross vs after the cross
It all depends on whether your eye catches the rays before they cross or after:
Big & right-side-up
The lens bends the rays inward, but not enough to cross before your eye — so they're still spreading apart, and your eye reads them as one big, upright picture.
Flipped upside down
The rays already crossed, so the top ray ends up on the bottom — the picture flips over.
3Your turn — shape the lens
Make the lens fatter and watch the rays cross sooner
Three straight rays come in from the left. Drag the slider to make the lens fatter or thinner and watch how hard it bends them — and where the focus point lands.
The lens bending three rays to a crossing point
4Now slide the bug across the focus
Same lens — drag the bug back and catch the flip 🐞
Now the lens stays put, so its focus point never moves. There's a little bug in front of it. The big test: slide the bug back and watch your eye's picture — does it flip right away, or hold steady until the bug crosses a certain line? Guess first — then drag it.
Guess WHEN it flips
You start backing the bug away from the lens. The picture flips upside down at some point — but when? The very instant you start moving it back, or only after it slides past one special hidden line?