Calculate the correct answer:
A person is prescribed a lens of power \(-\ 9\ D\) for seeing distant objects clearly and another lens of power \(+\ 4\ D\) for reading nearby objects comfortably.
a. Which vision defect is corrected by the \(-\ 9\ D\) lens? Name the type of lens used.
b. Which vision defect is corrected by the \(+\ 4\ D\) lens? Name the type of lens used.
c. Calculate the focal length of each corrective lens.
Focal length of \(-\ 9\ D\) lens is \(m\)
Focal length of \(+\ 4\ D\) lens is \(m\)
[Note: Enter the answer upto two decimal places.]