A method can include emitting, from a light source, directional light through fuel contained in a fuel tank, and determining a refraction angle of the directional light after the directional light passes through an interface with the fuel. The method can further include determining, by a processing device, an index of refraction of the fuel based on the determined refraction angle, and determining, by the processing device, a density of the fuel based on the determined index of refraction of the fuel.