Numerical answer · +4 / −1
A convex lens has power 5 D. Its focal length in cm is:
Answer and solution
Answer 20
Solution
Physics sir · 3 steps
Power is just one over focal length in metres, so flip 5 and then turn metres into centimetres.
- What power means. The power of a lens is the reciprocal of its focal length, with the focal length measured in metres. One dioptre is 1 m⁻¹.
P = 1/(f (in m)) ⇒ f = 1/P = 1/5 m
- Convert to cm. f = 0.2 m = 0.2 × 100 cm = 20 cm. The power is positive, which matches a convex (converging) lens.
- So. The focal length is 20 cm.
The common slip: 0.2. Stopping at f = 1/5 = 0.2 leaves the focal length in metres. The question asks for centimetres, so multiply by 100.
The idea to keep. P = 1/f works only with f in metres. If the focal length is in centimetres, use P = 100/f instead.
Answer from the NTA final answer key. Solution by PrepWiser.