For the same instantaneous geometry: a concave mirror of focal length 20 cm moves right at 2 cm/s; an object 30 cm to its left moves right at 5 cm/s. If the image moves opposite to the object's relative motion in the mirror frame, find its ground-frame velocity magnitude in cm/s.
Physics · Geometrical optics · spherical mirror
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Question

For the same instantaneous geometry: a concave mirror of focal length 20 cm moves right at 2 cm/s; an object 30 cm to its left moves right at 5 cm/s. If the image moves opposite to the object's relative motion in the mirror frame, find its ground-frame velocity magnitude in cm/s.

Physics · Geometrical optics · spherical mirror

G1 Physics · Accepted Answer

Relative object speed is +3 cm/s toward the mirror. Image relative velocity is −12 cm/s in the chosen axis sense. Adding mirror velocity +2 gives −10 cm/s; magnitude 10 cm/s.