Question

The radii of curvature of convexo - concave lens are 0.04 m and 0.05 m respectively. If its R.I. is 1.5. The focal length of the lens is

• A. $0.4m$
• B. $-0.4m$
• C. $-0.2m$
• D. $0.2m$

Answer 1

The correct option is B $-0.4m$

$\frac{1}{f}=(1.5-1)(\frac{-1}{0.04}+\frac{1}{0.05})$

$\frac{1}{f}=\frac{-0.01}{\left(0.04\right)\left(0.05\right)}\times \frac{1}{2}$

$f=-4\times 0.05\times 2$ $=-0.4m$