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
Two image types
The object's distance from the lens changes whether the emerging rays meet on the far side or only appear to come from a point on the near side:
Big & right-side-up
With the object inside the focal distance, the emerging rays spread apart. Tracing them backward gives an upright virtual image on the object's side.
Flipped upside down
With the object beyond the focal distance, rays from its top meet below the axis on the far side. This real image is inverted.
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 compare the image size
At twice the focal distance, how big is the image?
Keep the lens fixed. A new test puts the bug exactly two focal lengths from it. The focus marks tell you that distance. Predict the image's height before tracing the rays.
Guess before you trace the rays
With the bug two focal lengths away, will the image be taller than the bug, shorter, or the same height?