A prescription for new eyeglasses reads -4.50 D. What is the focal length and what kind of lens is it?

1. The focal length and power of a diverging lens are both defined to be negative. 2. A power of -4.50 D therefore already announces a diverging lens. 3. The focal length is f = 1 divided by P = 1 divided by (-4.50 D). 4. Substituting, f = -0.222 m, which is -22.2 cm. 5. Keep the minus sign: dropping it would report a converging lens and reverse the whole image analysis. 6. The -4.50 cm option simply copies the power figure and gives it the wrong unit. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_