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Applet by Fu-Kwun Hwang
The path of a ray of light between two points is the path that minimizes the travel time.
This java applet lets you visualize the above principle.
There are two regions of different colors (one for air and one for water). The white dot is the light source. The program shows all possible paths for light emitted from the point source, reaching the air/water interface and then all heading towards the blue/green dots.
The real path which arrive at either blue or green dot is the path that has the smallest traveling time, shown by the white path after the animation completes.
Instructions on how to use the animation below:
Click the left mouse button toggle for more information.
(The ratio of those numbers shown in yellow is related to the index of refraction!)
You can click white/blue/green dots and drag the mouse button to move it around.
Click the right mouse button to pause the animation, and click it again to resume.
Clicking near the label 'air/water' will switch the mediums.
Right click the top left corner to show all the real paths for light emitted from the point source. (Click it again to toggle between possible paths and real paths.)