Crystalline diffraction problem

[furtivefelon]

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 a lot!

 

[member 553083]

The equation relating the interplanar spacing and density together is Bragg's law. It states that the wavelength of the x-rays used in the experiment can be calculated by 2dsin- = m+, where m is the order of the diffraction pattern, d is the interplanar spacing, and is the glancing angle. In order to answer (a), you will need to calculate the wavelength of x-rays using the equation above. In order to answer (b), you can explain that visible light has a much longer wavelength than x-rays, making it less effective at diffracting off of crystalline structures and thus not suitable for obtaining a diffraction pattern.