When light travels from one medium to another with a different refractive index, which of the following will change?

$A$ light of wavelength $600 \ nm$ in air enters a medium of refractive index $1.5$. Inside the medium :

What is refraction of light?

The speed of light in a medium is $200 \times 10^8 \text{ cm/s}$. The refractive index of the medium is . . . . . . $(c = 3 \times 10^8 \text{ m/s})$.

Clarify the meanings of optical density and mass density.

The optical path of a particular ray of light that has travelled a distance of $3 \ cm$ in flint glass is the same as that of a ray travelling a distance of '$x$' $cm$ through another medium. The value of '$x$' is [refractive index of flint glass $= 1.6$, refractive index of another medium $= 1.25$]. (in $cm$)