# Which formulas are appropriate for calculating intensity?

1. Aug 4, 2017

### Ekullabran

1. The problem statement, all variables and given/known data
Monochromatic plane waves of wavelength 537nm are incident on two narrow slits and produce an interference pattern on a wall 3.1m away. The slits are spaced 26 micrometers apart and are 5.7 micrometers wide each. What is the ratio of intensity to that of the central fringe a distance of 35.5mm from the central fringe?

2. Relevant equation

I=I0(cos(∏(d/(λL)z))

3. The attempt at a solution

I tried using this formula to find the intensity of the central fringe and also the intensity of the fringe 35.5mm from the central fringe, however, I could not determine what I(sub)naught was.

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2. Aug 4, 2017

Your formula is correct, as you have it in the link. You left out the exponent of 2 when you typed it out. They simply want you to compute the ratio $\frac{I}{I_o}$. You should be able to do that using your formula. (Usually the letter $x$ is used instead of $z$ in this formula. In any case, $z=35.5$ mm). You should be able to figure out what $d$, $L$ and $\lambda$ are.

Last edited: Aug 4, 2017
3. Aug 4, 2017

### Ekullabran

That makes so much sense! Thanks a lot for your time!