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 24 - Optical Instruments - Exercises and Problems - Page 714: 42

Answer

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Work Step by Step

First, we need to find the first image created by the lens. We can use the thin lens formula $$\dfrac{1}{f_1}=\dfrac{1}{s_1}+\dfrac{1}{s'_1}$$ Hence, $$s_1'=\left[ \dfrac{1}{f_1}-\dfrac{1}{s_1} \right]^{-1}$$ Plug the known; $$s_1'=\left[ \dfrac{1}{10}-\dfrac{1}{5} \right]^{-1}=\bf -10\;\rm cm$$ This means that the image is still on the left side of the lens at its focal point. The height of this image is given by $$m=\dfrac{-s_1'}{s_1}=\dfrac{h_1'}{h}$$ Hence, $$h_1'=\dfrac{-s_1'h}{s_1}$$ Plug the known; $$h_1'=\dfrac{-(-10)(1)}{5}=\bf 2\;\rm cm$$ It is an upright virtual image. Now this image is the object of the mirror where $s_2=10+5=15$ cm, and $h_2=h_1'$. So by the same approach, $$s_2'=\left[ \dfrac{1}{f_2}-\dfrac{1}{s_2} \right]^{-1}$$ Plug the known; $$s_2'=\left[ \dfrac{1}{-30}-\dfrac{1}{15} \right]^{-1}=\bf-10\;\rm cm$$ This means that the image is behind the mirror. The height of this image is given by $$h_2'=\dfrac{-s_2'h_1'}{s_s}=\dfrac{-(-10)(2)}{15}=\bf \frac{4}{3}\;\rm cm$$ This means it is an upright virtual image too. This image is now the object for the lens where $s_3=5+10=15\;\rm cm$, so $$s_3'=\left[ \dfrac{1}{f_1}-\dfrac{1}{s_3} \right]^{-1}$$ $$s_3'=\left[ \dfrac{1}{10}-\dfrac{1}{15} \right]^{-1}=\bf 30\;\rm cm$$ This means that this final image is 30 cm to the left from the lens. And its height is then given by $$h_3'=\dfrac{-s_3'h_2'}{s_3}=\dfrac{-(30)(\frac{4}{3})}{15}$$ $$h_3'=\bf -2.67\;\rm cm $$ Thus, the final image is inverted and its height is 2.67 cm. It is at a distance of 30 cm to the left from the lens.
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