A magnifying glass held in sunlight concentrates the light to a small bright spot 8.00 cm from the lens. What are its focal length and its power?

1. The Sun is so far away that its rays are nearly parallel when they reach the Earth. 2. Rays entering a converging lens parallel to its axis cross at the focal point, so the bright spot is at the focal point and f = 8.00 cm. 3. Power is P = 1 divided by f, with f in metres, so first convert: 8.00 cm = 0.0800 m. 4. Substituting, P = 1 divided by 0.0800 m = 12.5 D. 5. Leaving f in centimetres gives 0.125 D, which is the commonest slip in this calculation. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_