A) 15 cm (concave)
B) 15 cm (convex)
C) 10 cm
D) 0
Correct Answer: A
Solution :
Since, the person is not able to see distant objects beyond \[10\,\,cm\] so, he should use concave lens in his spectacles. Concave lens will form the image of object distant \[30\,\,cm\], at \[10\,\,cm\] it means, \[u=-30\,\,cm,\,\,v=-10\,\,cm\] \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] \[=\frac{1}{-10}-\frac{1}{-30}\] \[=-\frac{2}{30}\] or \[\frac{1}{f}=-\frac{1}{15}\] So, \[f=-15\,\,cm\] Minus sign signifies that lens used is concave.
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