Answer
$f=-80cm$
Work Step by Step
We are asked to find the focal length, $f$. We can use equation 34-10 with the values given in the text and following the sign conventions described in the text.
$\frac{1}{f}=(n-1)(\frac{1}{r_1} - \frac{1}{r_2})$
Solving for $f$, we obtain:
$f=((n-1)(\frac{1}{r_1} - \frac{1}{r_2}))^{-1}$
$f=((1.5-1)(\frac{1}{\infty} - \frac{1}{40cm}))^{-1}$
$f=-80cm$
