An object is held 25.0 cm from a lens of power -4.00 D, of the kind used to correct short sight. What magnification results?

1. A negative power means a diverging lens, so the focal length is negative: f = 1 divided by (-4.00 D) = -0.250 m = -25.0 cm. 2. The object distance is positive: do = 25.0 cm. 3. Use di = f do divided by (do minus f). 4. Substituting with units, di = (-25.0 cm times 25.0 cm) divided by (25.0 cm minus (-25.0 cm)). 5. The numerator is -625 cm squared and the denominator is 50.0 cm, so di = -12.5 cm. 6. The magnification is m = minus di divided by do = minus (-12.5 cm) divided by 25.0 cm = +0.500. 7. Positive and less than one means upright and half size, which is what a diverging lens always gives. 8. Answering -0.500 forgets that the two minus signs cancel, and would wrongly report an inverted image. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_