Wet van Snellius
When light crosses from one medium to another of a different density, its speed and wavelength change. Where it strikes the new surface at an angle, the transmitted wave is refracted -- it changes direction.
More information:
http://poppersdreamland.blogspot.com/2012/02/snells-law-1.html
Drag L into the medium with a higher Index of Refraction to view reflected vector R.
The angle at which light fails to transmit due to the difference in density is the critical angle.
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This is an old worksheet! Improved versions:
Refraction: http://www.geogebratube.org/material/show/id/112931
Vector intersections: http://www.geogebratube.org/student/b74031#chapter/761
Light Properties (book) :http://tube.geogebra.org/material/show/id/128472