diff --git a/src/4_sommerfeld_model.md b/src/4_sommerfeld_model.md
index 5432712c1ae3728bdb3f2140b623ca7066cac5ca..038353d489325c51670fcabe54b93b02ecf8430d 100644
--- a/src/4_sommerfeld_model.md
+++ b/src/4_sommerfeld_model.md
@@ -133,7 +133,7 @@ Estates = kstates**2;
 
 fig = pyplot.figure();
 ax = fig.add_subplot(111);
-ax.fill_between(kfilled, Efilled, kf*kf);
+ax.fill_between(kfilled, Efilled, kf*kf, alpha=0.5);
 ax.plot([kf, kf], [0.0, kf*kf], 'k:');
 ax.plot(kstates, Estates, 'k--');
 ax.plot(kfilled, Efilled, linewidth=4);