A) \[-6.6D\]
B) \[+0.66\text{ }D\]
C) \[+6.6D\]
D) \[-0.66D\]
Correct Answer: D
Solution :
Key Idea: Convex lens is a converging lens while concave lens is a diverging one. Power of a lens is defined as reciprocal of focal length measured in metres. \[\therefore \] \[P=\frac{1}{f(in\,m)}D\]or\[P=\frac{100}{f(cm)}D\] Given, \[{{f}_{1}}=+75\,cm,{{f}_{2}}=-50\,cm\] \[\therefore \] \[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] \[=\frac{1}{75}-\frac{1}{50}\] \[=\frac{2-3}{150}=-\frac{1}{150}\] \[\Rightarrow \] \[f=-150\,cm\] \[\therefore \] power\[=\frac{100}{f}=-\frac{100}{150}=-0.66D\] Note: Since, focal length of combined lens is negative, it behaves as a diverging lens.
You need to login to perform this action.
You will be redirected in
3 sec
