A simple measurement of the refractive index of a liquid has been performed using a spherical lens. This lens is a spherical shape flask containing the liquid under study. Parallel pointer laser beams are directed to the lens to obtain the back focal length of the lens. The focal point is determined visually or using the Tracker software to analyze the photo. The back focal length is investigated related to the radius of curvature of the lens and the liquid under study. The refractive index at [lambda] = 635 nm resulted from the photo analysis for water, ethanol and vegetable oil are 1.34 ± 0.03, 1.38 ± 0.05, 1.47 ± 0.06, respectively. This simple experiment is suitable for teaching.