Bragg Diffraction: Calculate 1st & 2nd Order Angles

[Mohammed Shoaib]

The spacing between atomic planes in a crystal is 0.120 nm . 15.0 keV x rays are diffracted by this crystal What is the angle of first-order diffraction?What is the angle of second-order diffraction?My question is that the manual says to use 2dcosΘ=mλ instead of 2dsinΘ=mλ( bragg diffraction condition). Why?

 

[Zypheros_Knight]

Can you illustrate with a diagram?

 

[lychette]

The spacing between atomic planes in a crystal is 0.120 nm . 15.0 keV x rays are diffracted by this crystal What is the angle of first-order diffraction?What is the angle of second-order diffraction?My question is that the manual says to use 2dcosΘ=mλ instead of 2dsinΘ=mλ( bragg diffraction condition). Why?

With Bragg diffraction the angle used in the analysis is the angle between the crystal surface and the diffracted beams,...NOT the angle between the normal and the diffracted beams (you are probably more familiar with that one...Sin theta)
The diffraction is caused by reflection of X rays from the surface of the crystal not transmission (which is usually the case with light and transmission gratings)