where formula_3 is the width of the lens, changing in each point with a shape that is the same of formula_1 because formula_5 and "n" are constants. In other words, in order to get a focusing effect we just have to introduce a phase change of such a shape, but we are not obliged to change the width. If we leave the width "L" fixed in each point, but we change the value of the refractive index formula_6 we will get exactly the same effect, but with a completely different approach.