How can an object be a specular reflector for some electromagnetic waves yet be diffuse for others?
specular reflectors usually operate at the surface boundry atoms, in which electrons absorb the light wave energy, then reradiate such that the law of reflection holds.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Diffuse_reflection.gif/375px-Diffuse_reflection.gif
Diffusion occurs, when light passes thru the surface boundry, and there is not a coherent boundry that is reflecting in a uniform angle.
To understand how an object can act as a specular reflector for some electromagnetic waves while being diffuse for others, we need to delve into the concepts of specular reflection and diffuse reflection.
Reflection is the change in direction of waves when they encounter a surface. Specular reflection occurs when waves hit a smooth surface and bounce off in a predictable manner, with the angle of incidence being equal to the angle of reflection.
On the other hand, diffuse reflection occurs when waves hit a rough or irregular surface and scatter in various directions. The incident angle is not preserved, and the waves are scattered randomly.
Now, coming to your question, an object can behave as a specular reflector for a specific range of electromagnetic waves if its surface is smooth and the wavelength of those waves is similar to the size of irregularities on the surface. In this case, the waves hit the smooth surface and reflect in a predictable manner, following the laws of specular reflection.
However, for electromagnetic waves with different wavelengths, if the surface of the object is rough or irregular on a larger scale compared to the wavelength of those waves, the waves will encounter multiple irregularities and scatter in various directions. This scattering results in diffuse reflection.
To summarize, an object can act as a specular reflector for some electromagnetic waves when the surface is smooth and the wavelength is similar to the size of surface irregularities. It can be diffuse for other waves when the surface is rough or irregular on a larger scale compared to the wavelength of those waves.