Apparent distance of the light source changes based on fluid depth for fluids with index of refraction greater than air (relationship is inverted if index is less than air).

Refraction angles can be determined with Snell’s law with known indices of refraction of the 2 media (n1sin(θ1) = n2sin(θ2)). In the case of second transition back into the original medium, the relationship θ3 = θ1 can be derived, hence the labeling of the third angle as θ1 in this diagram. L = fluid depth, I = apparant distance of object. h0 = distance between object and top of fluid, h1 = distance between bottom of fluid and measurement plane.