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A glass slab has a refractive index of 1.5. If the speed of light in vacuum is $3 \times 10^8\ \text{m s}^{-1}$, what is the speed of light inside the glass?
A$1.5 \times 10^8\ \text{m s}^{-1}$
B$3.0 \times 10^8\ \text{m s}^{-1}$
C$2.0 \times 10^8\ \text{m s}^{-1}$
D$4.5 \times 10^8\ \text{m s}^{-1}$
Answer & Solution
Correct answer: C. $2.0 \times 10^8\ \text{m s}^{-1}$
1. Refractive index relates speed in vacuum to speed in medium: $\mu = \dfrac{c}{v}$.
2. Rearrange for $v$: $v = \dfrac{c}{\mu}$.
3. Substitute: $v = \dfrac{3 \times 10^8}{1.5} = 2.0 \times 10^8\ \text{m s}^{-1}$.
4. Light slows down in a denser medium, so the answer must be less than $c$.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Refraction of Light", p.24_
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