Question:
A slide projector has to project a $35 \mathrm{~mm}$ slide $(35 \mathrm{~mm} \times 23 \mathrm{~mm})$ on a $2 \mathrm{~m} \times 2 \mathrm{~m}$ screen at a distance of $10 \mathrm{~m}$ from the lens. What should be the focal length of the lens in the project?
Solution:
