9. If the power of a lens is -2.0D , then it means tha this is a concave lens. And what is the focal length o: this lens?

The power of a lens is defined as the reciprocal of its focal length (in meters). The negative sign in the power of the lens indicates that it is a concave lens.Given that the power of the lens is -2.0D, we can use the formula:Power (P) = 1 / Focal Length (f)Rearranging the formula to solve for the focal length, we get:Focal Length (f) = 1 / Power (P)Substituting the given power of -2.0D into the formula, we have:Focal Length (f) = 1 / (-2.0D)Calculating the focal length, we get:Focal Length (f) = -0.5 metersTherefore, the focal length of the lens with a power of -2.0D is -0.5 meters.