Guessing based on years old physics knowledge: Without the fluid, the surface of the stone appears foggy because it's diffusing light (like frosted glass.) The

Guessing based on years old physics knowledge: Without the fluid, the surface of the stone appears foggy because it's diffusing light (like frosted glass.) The fluid creates a smooth surface on top of that which doesn't diffuse light (like clear glass.) The refractive index refers to how much light bends when it enters a substance. To see clearly, the refractive index of the fluid and the stone should match as closely as possible. Otherwise, the surface of the stone would still be visible under the fluid.