## Conceptual Physics (12th Edition)

The criterion is that a surface is considered polished if the local height differences in the surface are less than about $\frac{1}{8}$ of the wavelength of the electromagnetic radiation. Thus, we see that a surface may be considered "smooth and polished" for long-wavelength waves, while simultaneously being "rough" for short-wavelength waves. For example, in Figure 28.11, the mesh on a radio telescope's parabolic dish is "smooth" to long-wavelength radio waves and can focus them, but is rough when viewed with visible-light waves. This is discussed on page 523, and shown in Figures 28.10 and 28.11.