Question

Can absolute refractive index be less than unity?

Answer 1

No never....

Abdolute refractive index is the ratio of velocity of light in air to the velocity of light in transparent medium like glass , water , diamond , kerosene etc...

refractive index of a medium
= velocity of light in air / Velocity of light in the medium

We know that velocity of light in vaccum is maximum and approximately equal to air...
so velocity of light in vaccum = velocity of light in air = 3 × 10^8 m / s
When light enters in any other transparent medium its velocity decreases...This is why light changes its ditection or bends when it travels from one transparent medium to other...This phenomena of bending the light is called refraction...

Note that : whenever we divide a number ( Dividend ) which is always greater than its divisor...Then we always get a quotiont greater than 1...

When we have a fraction in which Numerator greater than Denominore , we always get a quotiont greater than 1

For example : 2 / 1 , 11 / 3 , 4 / 3 , 23 / 7 ...etc Here if we find the quotient we always get greater than 1...

Now in the relation of refrative index... we have
refractive index of a medium
= velocity of light in air / velovity of light in medium
but
velocity of light in air always > velocity of light in a medium other air or vaccum

=> refrative index always > 1....