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 692: 68

Answer

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Work Step by Step

The author asks for an image on the wall, which means he needs a real image on the wall, so we have to use a converging lens. For lenses, $$\dfrac{1}{f}=\dfrac{1}{s}+\dfrac{1}{s'}\tag 1$$ The author told us to put the lens between the object and the wall which means that $$s+s'=2\;\rm m=200\;\rm cm\tag 2$$ Hence, $$s'=200-s$$ Plugging into (1), $$\dfrac{1}{f}=\dfrac{1}{s}+\dfrac{1}{200-s}$$ Plugging the known; $$\dfrac{1}{32}=\dfrac{1}{s}+\dfrac{1}{200-s}=\dfrac{200-s+s}{(200-s)s}=\dfrac{200}{200s-s^2}$$ Thus, $$ 200s-s^2 =(200)(32)=6400$$ $$s^2-200s+6400=0 $$ Hence, $$s_1=\color{red}{\bf 40}{\;\rm cm}, {\rm and}\;\; \;\;s_2=\color{red}{\bf 160}{\;\rm cm}$$ Therefore, from (2), $$s'_1=\color{red}{\bf 160}{\;\rm cm}, {\rm and}\;\; \;\;s'_2=\color{red}{\bf 40}{\;\rm cm}$$ Recalling that $m=-s'/s=h'/h$ So, $$m_1=\color{blue}{\bf- 4} , {\rm and}\;\; \;\;m_2=\color{blue}{\bf- \frac{1}{4}}$$ Where $h'=mh$ whereas $h=2$ cm. So that $$h'_1=\color{red}{\bf 8}{\;\rm cm}, {\rm and}\;\; \;\;h'_2=\color{red}{\bf 0.5}{\;\rm cm}$$ And the two images are inverted.
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