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CBSE Class 10 Science 2016 Term II Outside Delhi Set 3

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Question : 4 of 11
Marks: +1, -0
The refractive indices of glass and water with respect to air are 32\frac{3}{2} and 43\frac{4}{3} respectively. If speed of light in glass is 2×108 m/s2 \times 10^8 \text{ m/s}, find the speed of light in water.
Solution:  
Refractive index of a medium
= Speed of light in air  Speed of light in the medium = \frac{\text{ Speed of light in air }}{\text{ Speed of light in the medium }}
For glass,
32= Speed of light in air 2×108\frac{3}{2} = \frac{\text{ Speed of light in air }}{2 \times 10^8}
Speed of light in air =32×2×108=3×108 m/s= \frac{3}{2} \times 2 \times 10^8 = 3 \times 10^8 \text{ m/s}.
For water,
43= Speed of light in air  Speed of light in water \frac{4}{3} = \frac{\text{ Speed of light in air }}{\text{ Speed of light in water }}
Speed of light in water
=3×108×34=2.25×108 m/s= 3 \times 10^8 \times \frac{3}{4} = 2.25 \times 10^8 \text{ m/s}
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