BSL Physics Glossary - dioptre - definition
This is about the power of a lens which we call D. A thick lens has more power and has a short focal length. A thin lens has a much longer focal length. At the opticians you might have seen labels on the lenses, such as 2.5 D. What does it mean?
There is a formula: Power of lens = 1/focal length = dioptre
So 2.5D means that the focal length must be 0.4m:
Focal length = 1/D 1/2.5 = 0.4m