Question

According to the new Cartesian sign convention for lenses, the focal length of a concave lens is:

Options

• Negative

• Equal to the radius of curvature

• Positive

• Zero

Answer 1

Under the new Cartesian sign convention, the focal length of a concave lens is negative. This is because the parallel rays appear to diverge from the principal focus on the same side as the incident light, which is opposite to the direction of the refracted light.