What is the construction difference between a diffraction grating and a polarizing filter? I understand the functional difference between the two. If I rotate a diffraction grating the light of a laser does not dim, but it does with a polarizing filter. The diffraction grating is made of finely spaced lines that diffract the passing light. The polarizing filter is finely spaced lines that stop the passage of light that encounters the lines at the “wrong” angle. It seems to me that the polarizing filter and the diffraction grating are the same, but obviously they are not. How are they different? What in the construction makes different?
 A: A polarising filter contains long (polymeric) molecules arranged parallel to each other. They are special molecules (containing iodine atoms) that allow electrons to move back and forth along the molecule. Light with an oscillating electric field component parallel to the molecules will cause this back and forward electron movement, dissipating energy from the light and not allowing the component polarised thus to pass through. But electrons cannot pass from molecule to molecule, so there is much less absorption of energy from the component of light with its electric field at right angles to the molecules.
These long molecules are packed orders of magnitude closer together than a wavelength of light. But the spatial periodicity of a diffraction grating (naïvely speaking the distance between 'slits') is likely to be 2 or 3 wavelengths, to give wide spacing of the diffracted beams. A diffraction grating doesn't have these special molecules with 'free' electrons, as it is not required to polarise!
