I did a physics lab about diffraction grating where we had to determine the angular positions of all spectral lines of the helium discharge tube. I have 3 questions : 1. Why use a discharge tube instead of a light bulb? - I think it might be because you would have to break up the white light of the light bulb with a prism but im not sure if that's right. 2. Define order of diffraction and illustrate it. -I said that the order of diffraction is an integral multiple of wavelengths that allow for constructive interference. ( Im not sure if destructive interference comes in to play here ) 3. If "d" is spacing between 2 slits on a grating, how can we obtain grating constant? - I calculated the values of d but i don't know what grating constant is? I was thinking the number of slits in a certain area but I can't find any info anywhere? Please help. Thank you.