Refraction of light in oil causing a beaker to disappear. When light passes between media of different densities it refracts, or bends. The refractive index of a substance describes how light will propagate through a given medium. If an object immersed in a liquid has an refractive index that is different to that of the object, some of the light is refracted through the object and some is reflected back to the observer. However, if the object has exactly the same refractive index as the immersed object, such as the Perspex beaker and the vegetable oil here, the light will neither reflect nor refract at the interface between the object and the liquid it is immersed in. In that case, the object will appear to be invisible.