Meet the Lens
Convex gathers, concave scatters — watch a parallel beam decide
What this lesson covers
Why it matters
A lens is just a prism stack polished smooth — every zone bends light a little differently, and somehow they all agree on ONE point.
The idea in plain words
The lens vocabulary you'll use all chapter. Tap each term.
O · F · 2F
Sunlight (parallel rays) through a convex lens burns paper at 12 cm from the lens. What is f?
f = 12 cm — the classic sunlight method
- the anchor point: O — optical centre — a ray through it goes STRAIGHT
- one on each side: F — principal focus — where paraxial parallel rays meet (convex) or seem to come from (concave)
- the landmark: 2F — twice the focal length — the magic landmark where image size = object size
- cm or m: f — focal length — distance OF; shorter f = stronger lens
- Parallel rays meet at the principal focus
- So the burn point IS the focus
Predict first
A beam of parallel rays hits a CONCAVE lens. What happens?
Concave = diverging: rays fan out, and tracing them backwards they appear to start from the VIRTUAL focus on the incoming side.
- The rays spread out, as if coming from a point BEHIND the lens — correct
- They meet at a real focus in front
- They pass through unchanged
What you do
Fire a parallel beam through both lens types. Slide the focal length and watch the meeting point obey.
Check yourself
A convex lens is called converging because…
Convex = thicker at the CENTRE, gathering rays to a real focus. Image size depends on where the object stands.
The focus of a concave lens is called virtual because…
The diverging rays, traced backwards, meet at F on the incoming side — a real place, but no real light there.
Lens A has f = 10 cm, lens B has f = 25 cm. Which is stronger?
Power = 1/f. The lens that pulls rays to a focus SOONER is doing more bending.
- it bends parallel rays together through the focus — correct
- it is thicker at the edges
- it always makes bigger images
- no light actually passes through it — rays only APPEAR to come from it — correct
- it doesn't exist
- it moves around
- A — shorter focal length bends light harder — correct
- B — longer reach means more power
- Equal — both are convex