DESCRIPTION
Vintage diagram showing the refraction of a ray of light by a prism [url=http://www.istockphoto.com/file_search.php?action=file&lightboxID=10203398][img]http://www.walker1890.co.uk/istock/istock-mach.jpg[/img][/url][url=http://www.istockphoto.com/file_search.php?action=file&lightboxID=2789749][img]http://www.walker1890.co.uk/istock/istock-engraving.jpg[/img][/url]