Focal Length of a Spherical Mirror
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Rays arriving parallel to the axis of a spherical mirror reflect through a point halfway to the center of curvature — a small-angle result that makes mirror design beautifully simple. Grind a telescope mirror to a 4 m radius of curvature and you get a 2 m focal length. The rule underlies everything from shaving mirrors to the giant primaries of observatories (which are then tweaked into paraboloids to remove the residual spherical aberration).
Focal Length of a Spherical Mirror
Where
- = Focal length
- = Radius of curvature
Missing one of these? Work it out first, then come back
- Focal length — Thin Lens Equation, Lens Power in Diopters
- Radius of curvature — Centripetal Acceleration (a = v²/r), Centripetal Force (F = mv²/r)