Question

Determine the focal length of the lenses used in the spectacles.

Hint:

The focal length of a concave lens is negative, and it is determined by the inverse of the lens power. For \( P = -0.5 \, \text{D} \), the focal length is \( f = -2 \, \text{m} \).

Answer 1

The power of a lens is related to its focal length by the formula: \[ P = \frac{1}{f} \] where \( P \) is the power of the lens and \( f \) is the focal length. Given that the power of the lens is \( P = -0.5 \, \text{D} \), we can calculate the focal length as follows: \[ f = \frac{1}{P} = \frac{1}{-0.5} = -2 \, \text{m} \] Thus, the focal length of the lens used in the spectacles is \( \boxed{-2 \, \text{m}} \).