English: While not exact, the familiar formula for fringes when the screen is far away, the approximate formula, ${\displaystyle n\lambda =d\sin \theta }$, works surprisingly well for the screen close to the slits. Here, ${\displaystyle [d,y,L]=[321,274,404]].}$ The approximate formula for the first fringe is depicted as ${\displaystyle \lambda _{1}}$, which is shown as the length ${\displaystyle {\overline {CD}}}$. The actual wavelength is ${\displaystyle \lambda ={\overline {CE}}}$${\displaystyle =r_{2}-r_{1}}$, where point ${\displaystyle E}$ was found by creating the arc (dotted line) of length ${\displaystyle r_{1}}$, which swept around until it intersected with ${\displaystyle r_{2}}$. The approximate formula yields ${\displaystyle \lambda _{1}=180.1776}$, which is about 4% larger than the correct value of ${\displaystyle \lambda =173.6605}$.