A lens has a focal length of 25.0 cm. What is its power in diopters?

1. The power of a lens is P = 1 divided by f, and f must be given in metres, not in centimetres or millimetres. 2. Converting, 25.0 cm = 0.250 m. 3. Substituting, P = 1 divided by 0.250 m = 4.00 D. 4. One diopter is one inverse metre, so the unit follows straight from the conversion. 5. Using 25.0 cm directly gives 0.0400 D, a hundredfold error caused by skipping the conversion. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_