How can a surface be smooth/polished for some waves and not for others? (This is an important question, refer to the homework on WebAssign and question regarding whether a surface is smooth/polished for some electromagnetic waves verses others)

Depending upon the size of the irregularities on the surface and the wavelength of the surface a surface can be smooth for the waves of smaller wavelengths than the size of the irregularities.

Explanation:

Depending upon the size of the irregularities on the surface and the wavelength of the surface a surface can be smooth for some of the waves but not for the others.

More specifically, when a surface has irregularities of the size greater than or equal to the wavelength of a wave then the surface is rough for that particular wave and causes irregular reflection in the incident light.

Contrarily, when the surface has the irregularities less than the size of the wavelength then it reflects most of the light in a uniform manner.