A ray of light in air strikes the flat surface of a liquid, resulting in a reflected ray and a refracted ray. If the angle of reflection is known, what additional information is needed in order to determine the relative refractive index of the liquid compared to air?

Angle of refraction.

Explanation:

Refractive index is given as the ratio of the angle of incidence to the angle of refraction.

If angle of incidence is i and the refractive index of the incident medium is n₁, and if angle of refraction is r and refractive index of the refracting medium is n₂,

according to Snell's law, n₁ sin i = n₂ sin r,

relative refractive index = sin i / sin r