You can drag the light source to different place or change the index of refraction for the material.
If the index of refraction is larger than 1.414 (square root of 2) , all the light enter the fiber will be totally reflected at the boundary.

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Enjoy the fun of Physics! You can access this simulation online from
fiber optics
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan