Question

if the refractive index of light going from air to diamond is 2.42, what will be the refractive index for light going from diamond to air

Answer 1

given is refractive index of diamond with respect to air = 2.42

now the refractive index of air with respect to diamond = 1/refractive index of diamond with respect to air

= 1/2.42 = 0.41322

Answer 2

Refractive index=
$\frac{speed \: of \: light \: in \: first \: medium}{speed \: of \: light \: in \: second \: medium}$
$\frac{3 \times 10 {}^{8} }{1.24 \times 10 {}^{8} } = 2.419 = 2.42$
Refractive index of light going from diamond to air=
$\frac{1.24 \times 10 {}^{8} }{3 \times 10 {}^{8} } = 0.413 = 0.41$