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.<br /><br />Given that the power of the lens is -2.0D, we can use the formula:<br /><br />Power (P) = 1 / Focal Length (f)<br /><br />Rearranging the formula to solve for the focal length, we get:<br /><br />Focal Length (f) = 1 / Power (P)<br /><br />Substituting the given power of -2.0D into the formula, we have:<br /><br />Focal Length (f) = 1 / (-2.0D)<br /><br />Calculating the focal length, we get:<br /><br />Focal Length (f) = -0.5 meters<br /><br />Therefore, the focal length of the lens with a power of -2.0D is -0.5 meters.