Physics Young double slit experiment

[lucy1368]

Hello,

So the formula my teacher gave me to solve for constructive and destructive interference of light is d(Xm/L)=(m-1/2)lambda (destructive formula) where m must be a whole number. Is "m" the nodal line number from the central bright?

 

So for this problem, 638nm light is shining through a double slit. The distance from the first node(dark) to the third node is 4cm. Determine the slit seperation if the stance to the screen is 1.2m. 

 

So in the example, he solved that sicne D1 to D3 is 4cm, the distance between dark bands is 2cm. However, he used the constructive formula and plaggued in 2cm for the distance from Bo to B1. The constructive formual is d(Xm/L)=m*lambda. Why did he do this instead using the destructive formula which would be using the info from the question?The final answer is 38.3 micrometers btw. 

 

I have attached two diagrams; one to explain the eq'n and the other for the problem. 

 

Thank you so much!!

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[FizziksGuy]

The distance from max to max is the same as the distance from min to min, so either formula would work for the distance between them.