The speed of light in a vacuum is $3 \times 10^8$ m s$^{-1}$. Find its speed in glass of refractive index $1.5$.
A$1.5 \times 10^8$ m s$^{-1}$
B$4.5 \times 10^8$ m s$^{-1}$
C$2 \times 10^8$ m s$^{-1}$CORRECT
D$3 \times 10^8$ m s$^{-1}$
JUPEBPhysicsWaves and Optics
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Why the answer is C, and why the others tempt you.
$n = \dfrac{c}{v}$, so $v = \dfrac{c}{n} = \dfrac{3 \times 10^8}{1.5} = 2 \times 10^8$ m s$^{-1}$. Light slows down in a denser medium, so a value greater than $c$ is impossible.
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