CBSE Class 10 Science 2015 Term II Outside Delhi Set 1

Question : 4 of 36

Marks: +1, -0

The absolute refractive indices of glass and water are

4 \/ 3

and

3 \/ 2

respectively. If the speed of light in glass is

2 × 10^8 {\m} \/ {\ s}

, calculate the speed of light in (i) vacuum, (ii) water.

Solution:

(i) Given:

n_g=\;{4}/{3}, n_w=\;{3}/{2}, v_g=2 × 10^8 {\m} \/ {\ s}

We know that for glass,

n_g=\;{c}/{v_g}

∴

Speed of light in air or vacuum,

c\;=n_g v_g

\;=\;{4}/{3} × 2 × 10^8=2.67 × 10^8 {\m} \/ {\ s}

(ii) Also in water,

n_w\;=\;{c}/{v_w}

∴ \;\; v_w\;=\;{c}/{n_w}=\;{2.67 × 2 × 10^8}/{3}

Speed of light in water

=1.78 × 10^8 {\m} \/ {\ s}

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