Monochromatic light of wavelength λ illuminates a pair of thin parallel slits of width a separated by a distance d at normal incidence, producing an interference pattern on a distant screen. a) (10 points) Explain two modifications to the experiment which would cause the maxima (bright spots) of the diffraction pattern to move away from the center. b) (10 points) Explain two modifications to the experiment which would cause the minima (dark spots) of the interference pattern to move towards the center.

Explanation:

a ) If x be the position of n the bright fringe on the screen , following formula holds .

x = n (λD / d) ; λ is wave length , D is screen distance and d is slit separation .

If we increase the value of λ or  wave length, x will increase so central fringe along with all the fringes will shift away from the centre .

If we increase the value of D or screen distance , it will also increase x ,  so fringes along with central fringe  will shift away from the center.

b ) Fringes ,whether bright or dark , both shift together , either towards or away from the center .

So to move the dark spots towards the center , we need to do the opposite to what we did in the first case , ie decrease the wavelength or decrease the screen distance .

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