You are designing a lecture demonstration on diffraction gratings. You plan to u

Question

You are designing a lecture demonstration on diffraction gratings. You plan to use laser light (633 nm) and plan to shine the light through the grating onto a wall 9 m away from the grating. You want the separation between the central bright spot and the first order bright spot to be 1.0 m.

The scientific supply company offers gratings with the number of slits per millimeter.
How many slits per millimeter do you want your grating to have?
When you perform your demonstration, how many bright spots will be visible on the wall?

Explanation / Answer

In interfreence or diffraction pattern

the needed equation is Y = mLR/d---------------1

and d sin theta = mL--------------------2

where L = wavelgnth

m = order = 1,2,3,4, ......... for brigth bands

m = 1.5, 2.5, 3.5, 4.5, ......for dark bands

R is the distance from slit to screen

Y = disatnce from central spot to nth order fringe or fringe width

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so here Y = mLR/d

d = mLR/Y

d = 1* 633nm * 9/1

d = 5.697 um or 0.005697 mm

so no. of slits per mm is 1/d = 1/0.005697 = 175.53

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m = Yd/LR

m = 1* 5.697um/(633nm*9)

m = 1 order or 2 bright sopts on either sides are seen

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