Physics: Principles with Applications (7th Edition)

Published by Pearson
ISBN 10: 0-32162-592-7
ISBN 13: 978-0-32162-592-2

Chapter 25 - Optical Instruments - General Problems - Page 742: 66

Answer

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Work Step by Step

Equation 25–3 links the magnification to the 2 focal lengths. Solve for the focal length of the objective lens. $$M=-\frac{f_o}{f_e}$$ $$f_o=-Mf_e$$ The distance between the two lenses is the sum of the 2 focal lengths. Combine this with equation 25–3 and solve for the focal lengths. $$\ell=f_o+f_e=(1-M)f_e$$ The magnification for an astronomical telescope is negative. $$f_e=\frac{\ell}{1-M}=\frac{28cm}{1-(-7.5)}=3.3cm$$ $$f_o=-Mf_e=-(-7.5)( 3.3cm)=25 cm$$
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