A slide projector has a lens of focal length 100 mm and the slide sits 103 mm from the lens. How far away is the screen that receives a sharp image?

1. Both f and do are positive here, with f = 100 mm and do = 103 mm, and the object sits only just beyond the focal point. 2. Use di = f do divided by (do minus f). 3. Substituting with units, di = (100 mm times 103 mm) divided by (103 mm minus 100 mm). 4. The numerator is 10300 mm squared and the denominator is only 3 mm. 5. So di = 3433 mm, which is 3.43 m from the lens. 6. A tiny denominator is what makes the image land so far away, which is exactly how a projector fills a distant screen. 7. Answering 1.03 m or 10.3 m comes from mishandling the millimetre to metre conversion by a factor of ten. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_