It is easier to measure positions on the screen rather than angles. The position of the $p\text{th}$ dark fringe, at angle ${\theta }_p$, is $y_p=L\text{tan}{\theta }_p$, where $L$ is the distance from the slit to the viewing screen. Using Equation 33.20 for ${\theta }_p$ and the small-angle approximation $\text{tan}{\theta }_p\approx {\theta }_p$, we find that the dark fringes in the single-slit diffraction pattern are located at

A diffraction pattern is dominated by the central maximum, which is much brighter than the secondary maxima. The width $w$ of the central maximum, shown in Figure 33.15, is defined as the distance between the two $p=1$ minima on either side of the central maximum. Because the pattern is symmetrical, the width is simply $w=2y_1$. This is

The width of the central maximum is twice the spacing $\text{λ}\mathit{L}/\mathit{a}$ between the dark fringes on either side. The farther away the screen (larger $L$), the wider the pattern of light on it becomes. In other words, the light waves are spreading out behind the slit, and they fill a wider and wider region as they travel farther.

An important implication of Equation 33.22, one contrary to common sense, is that a narrower slit (smaller $a$) causes a wider diffraction pattern. The smaller the opening you squeeze a wave through, the more it spreads out on the other side.