Measuring the refractive index "n" of a substance or medium is part of every introductory physics course. Various approaches to determine this index have been developed over the years based on the different ways light reflects and transmits in the medium. In this paper, the authors would like to present a simple geometrical derivation of the refractive index of a transparent liquid that is obtained using a spherical concave mirror. This derivation relies mostly on Snell's law and the small-angle approximation. This method is based on measurements of the actual and apparent position of the centre of curvature of the mirror, when it is empty and filled with a liquid, respectively. The laws of reflection and refraction are essential to understand the physics behind this method. Students measuring indices of refraction of liquids using this technique are introduced gradually to the concepts of reflection and refraction of light, Snell's law and image formation by spherical mirrors. (Contains 3 figures.)