The Curved Mirror
Meet the concave mirror: P, F and C
What this lesson covers
Why it matters
Look into a polished spoon: the bowl shows you upside-down, the back shows you small and upright. Before explaining the magic, we name three points on a line.
The idea in plain words
The mirror's anatomy. Tap the points.
f = R ÷ 2
A concave mirror has radius of curvature 30 cm. Its focal length?
15 cm
- on the glass: pole P — the centre point of the mirror surface itself
- in space: centre of curvature C — centre of the sphere the mirror is a slice of
- the hot spot: focus F — where parallel rays meet — exactly halfway from P to C
- f = R/2 = 30/2
Predict first
Parallel rays (like sunlight) hitting a concave mirror…
The curve aims every reflected ray through one meeting point, F — which is why a concave mirror in sunlight can char paper.
- converge to one point — the focus — correct
- spread out in all directions
- bounce straight back
What you do
Pin the three landmarks on the axis: P on the mirror, F at the midpoint, C at the sphere's centre.
Check yourself
A concave mirror can set paper smoking at its focus because…
Energy spread over the whole mirror lands on a dot — concentration, not creation.
The focus of a concave mirror lies…
f = R/2 — the midpoint rule, worth one mark forever.
The inside of a shiny spoon behaves as a…
Curved toward you = concave — hence the upside-down face at breakfast.
- it concentrates all the sunlight it catches into that one point — correct
- mirrors create heat
- paper attracts light
- midway between the pole and the centre of curvature — correct
- at the centre of curvature
- behind the mirror
- concave mirror — correct
- convex mirror
- plane mirror