Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 23 - Ray Optics - Exercises and Problems - Page 693: 74

Answer

$ 1.16\;\rm mm/s$

Work Step by Step

We know that the lateral magnification is given by $$m=-\dfrac{s'}{s}\tag 1$$ We also know that the longitudinal magnification is given by $$M= \dfrac{ds'}{ds}\tag 2$$ See problem 82 in your textbook in this chapter, it refers to the same $M$. For the thin lens, $$\dfrac{1}{s}+\dfrac{1}{s'}=\dfrac{1}{f}$$ Solving for $s'$, $$s'=\left[ \dfrac{1}{f}-\dfrac{1}{s} \right]^{-1}=\left[ \dfrac{s-f}{fs} \right]^{-1}$$ $$s'=\dfrac{fs}{s-f} \tag 3$$ Plugging $s'$ from (3) into (1), $$m=-\dfrac{ 1}{s}\cdot \dfrac{fs}{s-f}$$ $$m=-\dfrac{ f}{s-f} \tag 4 $$ Plugging $s'$ from (3) into (2), $$M= \dfrac{d }{ds}\left[\dfrac{fs}{s-f}\right]=\dfrac{f(s-f)-fs}{(s-f)^2} $$ $$M= \dfrac{fs-f^2-fs}{(s-f)^2} =-\dfrac{f^2}{(s-f)^2}$$ $$M= -\left[\dfrac{f }{(s-f)}\right]^2=-m^2\tag{From (4)}$$ $$\boxed{M= -m^2}\tag 5$$ Now we are given that the speed of the object is 5 m/s which means that $$v_{\rm object}=\dfrac{ds}{dt}=5\;\rm m/s\tag 6$$ From (2) and (5), $$M=\dfrac{ds'}{ds}=-m^2\tag 7$$ Now we need to find $v_{\rm image}=ds'/dt$, $$v_{\rm image}=\dfrac{ds'}{dt}=\dfrac{ds'}{ds}\dfrac{ds}{dt}$$ Plugging from (6) and (7), $$v_{\rm image}=-m^2(5)=-5m^2$$ Plugging $m$ from (4), $$v_{\rm image}= -5\left[ -\dfrac{ f}{s-f} \right]^2$$ when the object (player) is at 10 m in front of the lens, $$v_{\rm image}= -5\left[ -\dfrac{ 150\times 10^{-3}}{10-(150\times 10^{-3})} \right]^2$$ $$v_{\rm image}= \color{red}{\bf -1.16}\;\rm mm/s$$ The negative sign means that the image is moving toward the lens while the object is moving away from the lens.
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