What is the focal length of a 2D lens when the formula F=1/D is applied?

• A. 39.37 inches
• B. 20 inches
• C. 45 inches
• D. 30 inches

Answer: (A)

To determine the focal length of a 2D lens using the formula ( F = \frac{1}{D} ), it is essential to understand that ( F ) represents the focal length, and ( D ) refers to the optical power of the lens measured in diopters. In this instance, a "2D lens" indicates that the optical power ( D ) is 2 diopters.

Using the formula:

[ F = \frac{1}{D} ]

Substituting ( D = 2 ):

[ F = \frac{1}{2} = 0.5 \text{ meters} ]

To convert this focal length into inches, we use the conversion factor where ( 1 \text{ meter} ) is approximately ( 39.37 \text{ inches} ):

[ F = 0.5 \text{ meters} \times 39.37 \text{ inches/meter} \approx 19.685 \text{ inches} ]

This result is closest to 20 inches. Therefore, when applying the provided formula and understanding the units, the calculated focal length accurately aligns with the value measured in inches for a lens with an optical power of