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## Homework Statement

A light source shines light of wavelength 490nm onto a pair of slits separated by 0.44m. Calculate the angular location and the location in cm of the second order dark fringes on a screen 1.4m from the slit

## Homework Equations

Xn = (n-1/2)L lambda/d

delta x = L lambda/d

## The Attempt at a Solution

So I understand how to find the angular location, but I don't understand why only one method for finding the location works. The method which works is using the formula containing Xn which gives the answer of 0.23cm. I don't understand why it is not possible given that this is double slit to use delta X = L lambda/d then multiply that answer by 2 to get your answer as it gives a different value.

X

_{2}=(2-1/2)(1.4)(4.9x10

^{-7})/0.44x10

^{-3}= 0.23cm (the answer in the back of the textbook)

Using ΔX = Lλ/d

ΔX = (1.4)(490x10

^{-9})/0.44x10

^{-3}

= 0.0015

as it is to the second dark fringe, shouldn't 0.0015x2 get me the same answer as my previous Xn calculation?

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