KVPY SA Exam 2015 Question Paper

Question : 25 of 80

Marks: +1, -0

A point source of light is moving at a rate of 2 cm-s

^{-1}

towards a thin convex lens of focal length 10 cm along its optical axis. When the source is 15 cm away from the lens the image is moving at

Solution:

u = – 15cm,
f = + 10 cm

1/v-1/u=1/\f⇒v={\fu}/{u+\f}

v={(+10)(-15)}/{-15+10}

v = + 30 cm

{dv}/{dt}=v^2/u^2{du}/{dt}

{dv}/{dt}=({+30}/{-15})^2

(+ 2 cm/s)

{dv}/{dt}=+8

cm/s (away from lens)

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