There is one problem in the problem set i don't know how to approach it.. Problem: While an artificial diffraction grating is used to study a spectrum of a light beam, electromagnetic waves with wavelengths much less than the wavelength of the visible light are used to study the natural diffraction grid, namely a crystalline structure. Electromagnetic waves in this range are called the x-rays. The condition for constructive interference is given by Bragg’s law: 2dsin- = m+, m = 1, 2, 3, … If the wavelength and diffraction angle are measured, the Bragg equation can be used to calculate the spacing between atomic planes. In our problem, the crystalline of NaCl with density D = 2.16 g/cm3 is exposed to x-rays with the glancing angle - = 60.0o. The reflected rays form the second order maximum. (a) Find the wavelength of x-rays used in the experiment. (b) Explain why the visible light cannot be used to obtain the diffraction pattern on crystals. i don't understand what equation relates the interplanar spacing and density together, or am i on the wrong track? Thanks alot!