A) 15cm (concave)
B) 15cm (convex)
C) 20cm
D) 40cm
Correct Answer: A
Solution :
Since the person is not able to see distant object beyond 10cm hence, he should use concave lens in his spectacles. Concave lens will help to form the image of object distant 30cm i.e., when, \[u=-30cm,\] \[\upsilon =-10cm\] From formula, \[\Rightarrow \] \[\frac{1}{f}=\frac{1}{\upsilon }-\frac{1}{u}\] \[\Rightarrow \] \[\frac{1}{f}=\frac{1}{-10}-\frac{1}{-30}=-\frac{2}{30}\] \[\Rightarrow \] \[\frac{1}{f}=\frac{1}{15}\] Hence, \[f=-15cm.\] Here minus sign signifies that lens used is in concave.You need to login to perform this action.
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