diff --git a/book/pages/waveguides_tir.ipynb b/book/pages/waveguides_tir.ipynb
index 54afccd..222f609 100644
--- a/book/pages/waveguides_tir.ipynb
+++ b/book/pages/waveguides_tir.ipynb
@@ -312,6 +312,20 @@
     "So for {term}`SOI` waveguides, the critical angle is ~24.5 degrees. For any angle greater than 24.5 degrees, there will be no transmitted wave and total internal reflection will occur."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Quick Check - Total Internal Reflection\n",
+    "\n",
+    "What is the critical angle for total internal reflection between lithium niobate (n=2.21) and air\n",
+    "(n=1.00)?\n",
+    "<details>\n",
+    "    <summary>Answer</summary>\n",
+    "    arcsin(1.00 / 2.21) = 26.9 degrees\n",
+    "</details>"
+   ]
+  },
   {
    "attachments": {},
    "cell_type": "markdown",