diff --git a/docs/sph3u7.md b/docs/sph3u7.md
index 4c78e78..1f93918 100644
--- a/docs/sph3u7.md
+++ b/docs/sph3u7.md
@@ -791,6 +791,8 @@ A light wave that is **coherent** is **monochromatic** (of the same wavelength)
 By using mafs involving rays along a nodal line, the separation, $s$ between **nodal/anti-nodal** lines on the distant surface can be related to the distance from the slits to the screen, $D$, the separation between the slits, $d$, and the wavelength $\lambda$.
 $$s=\frac{\lambda D}{d}$$
 
+The number of nodal lines depends on the ratio $\frac{d}{\lambda}$.
+
 **Dark fringe** positions on the screen are equally spaced at the same separation $s$ between the bright fringes. The perpendicular bisector with a length of $D$ is a **bright fringe**.
 
 <img src="/resources/images/fringe-slit-derive.png" width=700>(Source: Kognity)</img>