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A concave mirror has a focal length of 20 cm. A point object lies on…
Question
A concave mirror has a focal length of 20 cm. A point object lies on the principal axis 30 cm from the mirror and moves along the axis with a speed of 5 cm/s. Find the magnitude of the speed (in cm/s) of the image at this instant.
✓ Verified answer: 20checked by our engine — not a guess
Step-by-step solution
Mirror formula (Cartesian, concave f=-20, object u=-30): 1/v = 1/f - 1/u = -1/20 + 1/30 = -1/60, so v = -60 cm.
Longitudinal magnification of velocity: v_image = -(v^2/u^2)*v_object = -(60^2/30^2)*5 = -4*5 = -20 cm/s.
Magnitude = 20 cm/s.
Final answer20
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