$$ $$ $$ $$ $$ $$ $$ $$ $$ $\text{Extra close brace or missing open brace}$ $\text{Extra close brace or missing open brace}$ $$

Two-Slit Interference

The formula we want is $$y(n)=\frac{n\lambda L}{d}$$ where

• $\lambda$ is the wavelength of the light, usually measured in nanometers ($1\mathrm{nm}={10}^{-9}\mathrm{m}$)
• $d$ is the distance between the slits (often measured in millimeters: $1\mathrm{mm}={10}^{-3}\mathrm{m}$)
• $L$ is the distance from the slits to the screen where the pattern is projected
• $y$ is the distance from the center of the screen to the $n$th bright spot
• $n$ is the order of the bright spot you want to find

Possible pitfalls

• Confusing the different lengths in the problem
• Forgetting to convert all lengths to meters
• Forgetting that $d$ is the distance between the slits, not the width of each slit. (That would be $a$.)
• Confusing this with Diffraction Patterns, where $y$ is the position to the dark spots

Recommendation

Start the problem with a table like

$\lambda =$	___
$d=$	___
$L=$	___
$n=$	___
$y=$	___

fill in the values you know, and indicate the value that you need. Convert all of these to meters, substitute into the equation above, and then solve.