diff --git a/docs/sph3u7.md b/docs/sph3u7.md
index ec91551..4c78e78 100644
--- a/docs/sph3u7.md
+++ b/docs/sph3u7.md
@@ -701,7 +701,7 @@ $$E=E_0\cos\theta$$
 And so:
 $$I=I_0\cos^2\theta$$
 
-When **unpolarised light** passes through a polariser, the average result of $I\cos\theta$ is $\frac{1}{2}$, so the intensity of polarised light is **half** of the intensity of unpolarised light.
+When **unpolarised light** passes through a polariser, the average result of $I\cos^2\theta$ is $\frac{1}{2}$, so the intensity of polarised light is **half** of the intensity of unpolarised light.
 
 When unpolarised light reflects off of a **smooth non-metallic** surface it will be at least partially polarised.