Physics

Problem

For a concave lens, a teacher writes on the board: “Given

f=-20\,\text{cm}

and

u=+30\,\text{cm}

, the lens formula is

\displaystyle \frac{1}{v}=\frac{1}{f}+\frac{1}{u}

.” Spot the sign that has been written incorrectly and rewrite the lens formula with the correct signs only.