A point object 36 cm from a concave mirror moves at 10 cm/s making 53° with the axis. Its transverse image speed is 12 cm/s. Use sin53°=4/5. Find |f| in centimetres.
Physics · Geometrical optics · spherical mirror
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Question

A point object 36 cm from a concave mirror moves at 10 cm/s making 53° with the axis. Its transverse image speed is 12 cm/s. Use sin53°=4/5. Find |f| in centimetres.

G1 Physics · Accepted Answer

Object transverse speed=8, so |m|=12/8=1.5. For a real image, u=f(1+1/M)=5f/3. Thus f=3×36/5=21.6 cm.