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 lens power relates to focal length using:\[P = \frac{1}{f}\]where \( P \) is the power and \( f \) is the focal length.For \( P = -0.5 \, \text{D} \), the focal length is:\[f = \frac{1}{P} = \frac{1}{-0.5} = -2 \, \text{m}\]The spectacles' lens focal length is \( \boxed{-2 \, \text{m}} \).