A) \[-3.75\text{ }D\]
B) \[-10.0D\]
C) \[1.67D\]
D) \[5.0D\]
Correct Answer: A
Solution :
From the formula for the combination of the lenses \[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] \[\frac{1}{80}=\frac{1}{20}+\frac{1}{{{f}_{2}}}\] \[\frac{1}{{{f}_{2}}}=-\frac{1}{20}+\frac{1}{80}=-\frac{3}{80}\] \[{{f}_{2}}=-\frac{80}{3}=-\frac{0.8}{3}m\] \[P=\frac{1}{f}=-\frac{\frac{1}{0.8}}{3}=-3.75\]
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