$A$ person needs a lens of power $-2.5 \ D$ for correcting his vision. Name the defect of vision he is suffering from. Which lens will he be using for the correction? Also,find the focal length of the lens.

(N/A) The power of the lens is negative $(P = -2.5 \ D)$,which indicates that the person is using a concave lens.
Since a concave lens is used to correct myopia (nearsightedness),the person is suffering from myopia.
The focal length $f$ is calculated using the formula $f = 1 / P$.
Substituting the value: $f = 1 / (-2.5) = -0.4 \ m$.
Converting to centimeters: $f = -0.4 \times 100 = -40 \ cm$.
The negative sign indicates that the lens is concave.