# Change of a lens' focal point with wavelength. Refractive index wavelength independent

I was using a Comsol example for the focusing lens:

https://www.comsol.com/model/focusing-lens-57621

I made no changes to the model. The focal length of the lens is set to be 1000 um.

Initially, I was scanning the wavelength of the incident EM wave. The scan was done from 1 um to 9 um, see the attached pictures - there is also the equation solved by Comsol included in these pictures.

From the pictures, it is clear that the focal point of the lens is changing with the wavelength, even though the refractive index in the lens is wavelength independent - it is just artificially set to a some specific value.

I wish to ask you for a clarification of this effect, as I was not expecting it and was surprised to see it. My only explanation is that the usual "lens maker formula" (which states that if there is not wavelength dependent index of refraction, the focus should remain constant) is derived from the geometrical optics - eikonal equation, which in turn is an approximation for the wave equation for cases when the wavelength tends to zero.

In this case we are working in the MIR/NIR region, and maybe the eikonal equation is not valid anymore.