Question

From the ray diagram show, calculate the focal length of the concave lens.

Answer 1

$u=-60cm$

$f=20cm$

So from lens formula

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{v}+\frac{1}{60}=\frac{1}{20}$

$\frac{1}{v}=\frac{1}{30}$

$v=30cm$

hence focal length of concave lens is 30-10 = 20cm