# Focal Length, Wavelength Relationship

I have a cylindrical lens with a design wavelength of 587.6nm. The radius (S2) of the lens is 2mm, and the radius of the opposite face (S1), i assume is 0mm.

The focal length is given as 3.91mm and the back focal length is given as 1.4mm.

If I am using a wavelength of 905nm.

The question is how do I calculate the change in focal length of the lens due to the increase in wavelength. I have read many articles that seem to relate the lens equation and something called cauchy equation but I couldn't see anything that made sense to my non-optic way of thinking.

thank you in advance ddB