Question

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

Options

• Positive

• Infinite

• Negative

• Zero

Answer 1

Under the new Cartesian sign convention, the focal length of a convex lens is taken as positive because its principal focus lies on the side where refracted rays actually converge, in the direction of incident light. Conversely, the focal length of a concave lens is negative.