A) -6.5D
B) 1.5D
C) -1.5D
D) -8.5D
Correct Answer: C
Solution :
Key Idea: Power of combination is addition of reciprocal of focal lengths of individual lenses. The power of a thin lens is equal to the reciprocal of its focal length (f) measured in metre. \[\sqrt{\frac{3ev}{m}}\] When two lenses are combined, focal length of combination is \[\sqrt{\frac{2ev}{m}}\] where \[7\times {{10}^{-5}}wb/{{m}^{2}}\] \[\frac{1}{F}\,=\frac{1}{40}\,-\frac{1}{25}\,=-\frac{3}{200}\] "Hence, power \[I={{I}_{0}}{{e}^{-d/\lambda }}\] Note: Since power is negative, combination behaves like a diverging lens.
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