Out of the following, correct statement(s) is / are

a) the setting sun appears higher than as it really present due to atmospheric refraction

b) In glass red colour beam have less refractive index than violet colour light ${\mathrm{\mu }}_{\mathrm{r}}<{\mathrm{\mu }}_{\mathrm{v}}$because

    ${\mathrm{\lambda }}_{\mathrm{R}}>{\mathrm{\lambda }}_{\mathrm{v}}$ so velocity of red light is greater than violet light

c) refractive index of diamond is greater than glass ${\mathrm{\mu }}_{\mathrm{diamond}}=2.42 {\mathrm{\mu }}_{\mathrm{g}}=1.5$

d) refractive index of material for red colour light is always lesser than refractive index of

    violet.

Because ${\mathrm{\lambda }}_{\mathrm{R}}>{\mathrm{\lambda }}_{\mathrm{v}}\Rightarrow {\mathrm{V}}_{\mathrm{R}}>{\mathrm{V}}_{\mathrm{v}}$ in a given material

so ${\mathrm{\mu }}_{\mathrm{R}}<{\mathrm{\mu }}_{\mathrm{v}}$