The absolute refractive index of a medium is a measure of how much the medium slows down the speed of light compared to its speed in vacuum. It is denoted by the symbol "n". The absolute refractive index of medium a is given as 2.0, and the absolute refractive index of medium b is given as 1.5.

To calculate the speed of light in vacuum, we need to know that the speed of light in a medium is inversely proportional to the absolute refractive index of the medium. This means that as the absolute refractive index increases, the speed of light decreases.

The formula to calculate the speed of light in a medium is:
Speed of light in a medium = Speed of light in vacuum / Absolute refractive index of the medium

Let's calculate the speed of light in vacuum first:

Given:
Speed of light in medium b = 2 * 10^8 m/s

We can use the formula to find the speed of light in vacuum:
Speed of light in vacuum = Speed of light in medium b * Absolute refractive index of medium b

Substituting the values:
Speed of light in vacuum = (2 * 10^8 m/s) * (1.5)

Simplifying the expression:
Speed of light in vacuum = 3 * 10^8 m/s

Therefore, the speed of light in vacuum is 3 * 10^8 m/s.

Now, let's calculate the speed of light in medium a:

Given:
Absolute refractive index of medium a = 2.0

Using the formula:
Speed of light in medium a = Speed of light in vacuum / Absolute refractive index of medium a

Substituting the values:
Speed of light in medium a = (3 * 10^8 m/s) / (2.0)

Simplifying the expression:
Speed of light in medium a = 1.5 * 10^8 m/s

Therefore, the speed of light in medium a is 1.5 * 10^8 m/s.

In summary:
- The speed of light in vacuum is 3 * 10^8 m/s.
- The speed of light in medium a is 1.5 * 10^8 m/s.