Answer: dioptre.
- A watt
- B dioptre
- C lumen
- D candela
Correct answer: B. dioptre
Explanation: Power P = 1/f (f in metres). Its unit is the dioptre (D), where 1 D = 1 m<sup>-1</sup>.
Concave mirror ray construction: a ray parallel to the axis reflects through F, and a ray through F reflects parallel, locating the inverted real image. Image: Maxmath12, CC0, via Wikimedia Commons.
Concept context
Reflection, refraction, lenses, mirrors, total internal reflection.