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Diffraction grating 
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#1
Oct2905, 02:34 PM

P: 17

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. 


#2
Oct2905, 04:02 PM

P: 187

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 [tex]d sin\theta = n\lambda[/tex] 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. 


#3
Oct2905, 07:34 PM

P: 21

I'd like to add something in regards to #1, if you look at the grating equation, you'll realize that different wavelengths can actually have the same diffraction angle, given thier diffraction orders. For example, a 600nm first order ray will defract in the same angle as a 300nm second order ray.
This is a common problem with some gratings when trying to quantify the intensities, though I'm not sure how messy it would look in the experiment. As it has already been stated, the Helium wavelengths are spaced well apart. So in a matter of speaking, the Helium pattern will be more "cleaner" than the lightbulb pattern. 


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