1What is meant by the power of a lens? Write its formula and SI unit.
2How does the thickness of a convex lens affect its focal length and its power?
3Why is the focal length of a concave lens taken as negative?
4Differentiate between the real focus of a convex lens and the apparent focus of a concave lens.
5Calculate the power of a convex lens of focal length 25 cm.
6A person's spectacle lens has a power of -2 D. What kind of lens is it and what is its focal length?
7Two convex lenses have powers of +2 D and +5 D. Which has the shorter focal length, and which converges light more strongly?
8Why must the focal length be in metres when using \( P = \frac{1}{f} \)?
9The power of a glass lens does not change, but the power of the eye lens does. Explain why.