Fix equation for refracted rays of non-unit length

Resolves RayTracing#1644

Author: hollasch

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
 

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.