diff --git a/util/Vector.h b/util/Vector.h
index ebc944f..5fc9450 100644
--- a/util/Vector.h
+++ b/util/Vector.h
@@ -73,11 +73,11 @@ class Vector
         Vector refract(const Vector & target, double n1, double n2) const
         {
             const double n = n1 / n2;
-            const double cosI = dot(target);
+            const double cosI = -dot(target);
             const double sinT2 = n * n * (1.0 - cosI * cosI);
             if (sinT2 > 1.0)
                 return Vector(0.0, 0.0, 0.0);
-            return (*this) * n - target * (n * cosI + sqrt(1.0 - sinT2));
+            return (*this) * n + target * (n * cosI - sqrt(1.0 - sinT2));
         }
 
         Vector getPerpendicular() const