# PhysicsDiffraction

#### MermaidWonders

##### Active member
For what ratio of slit width to wavelength will the first minima of a single-slit diffraction pattern occur at $\pm 90°$?

The thing is, when I did it, I used the formula $sin\theta = \frac{n\lambda}{a}$, and used the fact that $m = 1$ and $\pm 90°$ to solve for $\frac{a}{\lambda}$. However, I don't know if we're supposed to plug in $-90°$ for $sin\theta$, because that would mean that our ratio of $\frac{n\lambda}{a}$ would be $-1$ as opposed to just $1$ (when $sin90°$ was plugged in)....

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#### topsquark

##### Well-known member
MHB Math Helper
For what ratio of slit width to wavelength will the first minima of a single-slit diffraction pattern occur at $\pm 90°$?

The thing is, when I did it, I used the formula $sin\theta = \frac{n\lambda}{a}$, and used the fact that $m = 1$ and $\pm 90°$ to solve for $\frac{a}{\lambda}$. However, I don't know if we're supposed to plug in $-90°$ for $sin\theta$, because that would mean that our ratio of $\frac{n\lambda}{a}$ would be $-1$ as opposed to just $1$ (when $sin90°$ was plugged in)....
More or less we can take the sign of $$\displaystyle sin( \theta )$$ to be "attached" to the n. A positive n describes the nth minima to the right of the central maximum and a negative n describes the nth minima to the left of the central maximum.

-Dan

#### MermaidWonders

##### Active member
More or less we can take the sign of $$\displaystyle sin( \theta )$$ to be "attached" to the n. A positive n describes the nth minima to the right of the central maximum and a negative n describes the nth minima to the left of the central maximum.

-Dan
OK, makes sense. So should I take the absolute value, since a negative ratio wouldn't be very meaningful in this context?

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#### topsquark

##### Well-known member
MHB Math Helper
OK, makes sense. So should I take the absolute value, since a negative ratio wouldn't be very meaningful in this context?
Yup.

Double check the angle on the |n| = 1 interpretation I gave you. I might have screwed up left and right. But in the long run, no, it doesn't really matter.

-Dan

#### MermaidWonders

##### Active member
Yeah, OK, makes sense. Thanks so much!