A myopic person can clearly see objects only up to 4 m and wants to see objects up to 20 m. Using $f = \dfrac{xy}{x-y}$, what focal length concave lens is needed?

1. Use $f = \dfrac{xy}{x-y}$ with $x = 4$ m and $y = 20$ m. 2. Numerator: $xy = 4 \times 20 = 80$. 3. Denominator: $x - y = 4 - 20 = -16$. 4. So $f = \dfrac{80}{-16} = -5$ m. 5. The negative sign confirms a concave lens is required for myopia. 6. A positive value would imply a convex lens, which is the wrong defect, so $+5$ m is a trap. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Solved Problems", p.35_