Describe how you find the focal length of a concave mirror.

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The Center of Curvature (CC) is the geometric center of the sphere from which the concave mirror was cut.  Traveling along that line towards the center of the mirror is the Focal Point (FP), and following that line to the mirror is the Vertex (V) or the center of the...

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The Center of Curvature (CC) is the geometric center of the sphere from which the concave mirror was cut.  Traveling along that line towards the center of the mirror is the Focal Point (FP), and following that line to the mirror is the Vertex (V) or the center of the mirror. The line describes all these points (CC, FP, and V) is the Principal Axis of the mirror.

To find the Focal Point, point the Principal Axis of the mirror towards a distant light source (such as the sun.)  Holding a sheet of paper in front of the mirror, move it closer to the Vertex until all the light rays converge on a bright, white spot (if there's enough solar energy, the rays will set the paper afire!) That determines where the Focal Point is. Simply measure from that point of convergence to the Vertex, which will give you the focal length of the mirror.

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