A) 18.2cm
B) 16.8cm
C) 17.5cm
D) 15.5cm
Correct Answer: C
Solution :
From lens formula \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\]where \[f\] is focal length, v the image distance and u the object distance. For convex lens \[\frac{1}{f}=\frac{1}{v}-\frac{1}{(-\mu )}=\frac{1}{v}+\frac{1}{u}\] Multiplying both sides by, u we get \[\frac{u}{v}+1=\frac{u}{f}\] \[\Rightarrow \] \[\frac{u}{v}=\frac{u-f}{f}\] or \[\frac{v}{u}=\frac{f}{u-f}\] or \[m=\frac{f}{u-f}\] According to question \[{{m}_{1}}=-{{m}_{2}}\] \[\therefore \] \[\frac{f}{{{u}_{1}}-f}=\frac{f}{f-{{u}_{2}}}\] \[\Rightarrow \] \[\frac{f}{20-f}=\frac{f}{f-15}\] \[\Rightarrow \] \[2f=20+15\] \[\Rightarrow \] \[f=\frac{35}{2}=17.5\,\,cm\]
You need to login to perform this action.
You will be redirected in
3 sec
