Question

Calculate the focal length of a lens of power –2.75 D.

Answer 1

Power −2.75 D indicates that lens is concave.

\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]

\[ \Rightarrow - \ 2.75 = \frac{100}{f \text{(in cm)}} \]

\[ \Rightarrow f = -\frac{100}{2.75} \]

\[ \therefore \ f = - \ 36.36 \ \text{cm} \]

Thus, the focal length of the lens is −36.36 cm.