The double convex lens is used to couple a laser beam of diameter $5\;\text{mm}$ into an optical fiber with a numerical aperture of $0.5$. The minimum focal length of the lens that should be used in order to focus the entire beam into the fiber is
- $1.44\;\text{mm}$
- $2.50\;\text{mm}$
- $4.33\;\text{mm}$
- $5.00\;\text{mm}$