In Vacuum Space the light has a constant speed, aprox=
.
However, depending on the medium and its Refraction Index (The resistance of the medium or material to the light that passes through) it changes the velocity.
Also is known that in liquids (for example, water) the refraction index is lower higher tnah in gas (air, for example).
So, in this problem the beam of light (1) passed through liquid, then with (2) the gas the refraction index decreased and the velocity increased. Then, the third medium is (3) opaque solid, in which all the beam of light is absorbed and refracted.