Question

Q.7 refractive index of glass is 1.65, what is the speed of light in glass?

Answer 1

according to me answer is
$1.8181 \times {10}^{8} m \div s$

Answer 2

$\Large \bf Given :$

$\bf Refractive \: index \: of \: flint \: glass, \: η_{g} = 1.65$

$\Large \bf To \: Find :$

$\bf Speed \: of \: light \: in \: glass, v_{g}$

$\Large \bf Solution :$

$\bf We \: know \: that \: speed \: of \: light \: in \: air, c=3×10⁸m/s$

Now, by formula :

$\bf η_{g} = \dfrac{c}{v_{g}}$

$\bf \implies v_{g} = \dfrac{c}{η_{g}}$

$\bf \implies v_{g} = \dfrac{3 \times 10^{8}m/s}{1.65}$

$\bf \implies v_{g} = \dfrac{3}{1.65}\times 10^{8}m/s$

$\bf \implies v_{g} = \dfrac{3 \times 100}{165}\times 10^{8}m/s$

$\bf \implies v_{g} = \dfrac{\cancel{3} \times 100}{\times{\cancel{165}}_{55}}\times 10^{8}m/s$

$\bf \implies v_{g} = \dfrac{100}{55}\times 10^{8}m/s$

$\bf \implies v_{g} = 1.8181... \times 10^{8}m/s$

$\bf \implies v_{g} = 1.82 \times 10^{8}m/s \: (approx.)$

$\bf \therefore Speed \: of \: light \: in \: glass, v_{g} = 1.82 \times 10^{8}m/s \: (approx.)$