First slide

Ray optics

Question

A car is fitted with a convex side -view mirror of focal length 20cm. A second car 2.8m behind the first car is overtaking the first car at relative speed of 15m/s. The speed of the image of the second car as seen in the mirror of the first one is

Difficult

A

1/15 m/s
B

10 m/s
C

15 m/s
D

1/10 m/s

Solution

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}, \frac{1}{20} = \frac{1}{v} - \frac{1}{280} \Rightarrow v = \frac{280}{15} cm V_{i} = [\frac{v^{2}}{u^{2}}] V_{0} V_{i} = {[\frac{280}{15 x 280}]}^{2} x 15; V_{i} = \frac{1}{15} {ms}^{- 1}$

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