1 dioptre is defined as the power ofalens focal length 1 metre. It is the S.I. unitof Power of lens. .:. 1D = 1 m-1. 51.Which one of the following materials be used to make a lens? glass, water, clay, carbon tetrachloride A lens allows light to pass through it. Since clay does not show such property, it cannot be used to make a lens.