Finding the separation of two slits

[asilvester635]

Homework Statement

How do I know what value of m to use when using the equation? Here we had to use m = 1 to find the answer.

Please refer to part 3 for the problem statement.

Homework Equations

Please refer to part 3 for the relevant equations.

The Attempt at a Solution

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[Orodruin]

The formula gives you the location of the bright fringes. You are given the distance between consecutive friges, not the position of a particular one.

 

[ehild]

Homework Statement

How do I know what value of m to use when using the equation? Here we had to use m = 1 to find the answer.

Please refer to part 3 for the problem statement.

Homework Equations

Please refer to part 3 for the relevant equations.

The Attempt at a Solution

View attachment 223112 [/B]

"Adjacent bright fringes are 4 mm apart" means that the the distance between the m+1-th bright fringe and the m-th one is 4 mm.
$y_m =m \frac{λL}{d}$ and
$y_{m+1} =(m+1) \frac{λL}{d}$
and $y_{m+1}-y_m= 4 mm$
Subtract the firs equation from the second one, what do you get?