From 53c6e393173e663cbae6ba6ebd43af12ed6868ea Mon Sep 17 00:00:00 2001
From: Steve Hollasch <steve@hollasch.net>
Date: Sat, 5 Oct 2024 13:29:20 -0700
Subject: [PATCH] Fix equation for refracted rays of non-unit length

Resolves #1644
---
 CHANGELOG.md                      | 3 ++-
 books/RayTracingInOneWeekend.html | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e3d9d49c..e4b90447 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,11 +1,12 @@
 Change Log / Ray Tracing in One Weekend
 ====================================================================================================
 
-# v4.0.2 (2024-10-01)
+# v4.0.2 (in progress)
 
 ### Common
 
 ### In One Weekend
+  - Fix    -- Fix equation for refracted rays of non-unit length (#1644)
 
 ### The Next Week
 
diff --git a/books/RayTracingInOneWeekend.html b/books/RayTracingInOneWeekend.html
index 8b9f651b..ed26c8a7 100644
--- a/books/RayTracingInOneWeekend.html
+++ b/books/RayTracingInOneWeekend.html
@@ -3310,7 +3310,7 @@
 
 If we solve for $\mathbf{R'}_{\bot}$ and $\mathbf{R'}_{\parallel}$ we get:
 
-  $$ \mathbf{R'}_{\bot} = \frac{\eta}{\eta'} (\mathbf{R} + \cos\theta \mathbf{n}) $$
+  $$ \mathbf{R'}_{\bot} = \frac{\eta}{\eta'} (\mathbf{R} + |\mathbf{R}| \cos(\theta) \mathbf{n}) $$
   $$ \mathbf{R'}_{\parallel} = -\sqrt{1 - |\mathbf{R'}_{\bot}|^2} \mathbf{n} $$
 
 You can go ahead and prove this for yourself if you want, but we will treat it as fact and move on.