Optics - Index of Refraction

A fast charged particle passes perpendicularly through a thin glass sheet of index of refraction 1.5. The particle emits light in the glass. The minimum speed of particle is 

A.  1/3 c
B.  4/9 c
C. 5/9 c
D. 2/3 c
E. c
(GR9277 #69)
Solution:

Index of refraction, nc/v
n = 1.5 = 3/2
v = c/n = 2/3 c

Answer: D

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