diff --git a/example02_threaded_raytracing/README.md b/example02_threaded_raytracing/README.md
index d5536df..c7862a1 100644
--- a/example02_threaded_raytracing/README.md
+++ b/example02_threaded_raytracing/README.md
@@ -1,4 +1,6 @@
 # Legal stuff
 
 Uses [glm](https://github.com/g-truc/glm) which is licensed under the
-["Happy Bunny License" or the MIT License](https://github.com/g-truc/glm/blob/master/copying.txt).
+["Happy Bunny License" or the MIT License](https://github.com/g-truc/glm/blob/master/copying.txt).  
+Uses [libpng](http://libpng.org/pub/png/libpng.html) which is licensed under the
+[PNG Reference Library License version 2](http://libpng.org/pub/png/src/libpng-LICENSE.txt).