## Diffraction grating

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?

 the helium is used instead of a light bulb because a certain element only emits certain wavelengths of light while the bulb would emit all wavelengths, and even if you were to split this with a prism you would still see all the colors and not specific ones. the order of diffraction is correct by what you said, since the n in the equation $$d sin\theta = n\lambda$$ refers to the order of diffraction, where n=1 is the first, n=2 is the second and so on. the grating constant k, is simply 1/d. d is the distance between slits and k, is the number of slits per unit length.