As power is equal to useful energy transferred per second, another way to calculate efficiency is to use the formula: \(efficiency = \frac{useful~power~transferred}{total~power~supplied}\) ...

\(power \ of \ a \ lens \ (D) = \frac{1}{focal \ length \ (m)}\) A lens has a power of -3.33 dioptres. Calculate the focal length and state what type of lens it is.