In another puzzle, a secret message on the inside of a bottle only became visible upon tilting the bottle and thus displacing the liquid. The core of this effect is the index of refraction. The secret number, which is made of glass is only visible under normal circumstances due to the way glass interacts with light. Particularly, the transition from air to glass is of vital importance. Whenever light travels from a medium with a certain index of refraction to an area with another, it is refracted. This is why shower glass is sometimes not see-through, as the rough surface scatters the light, making a coherent image impossible.
It is important to note and understand that this is only possible if the two media have different index of refraction. There are, however, multiple substances that have a similar index of refraction, so that break light in the same way. For instance, the oil used in this experiment has the same index of refraction as glass. Therefore, when the glass number is submerged in this oil, there are no longer any light effects and the number is no longer visible. As soon as the number is no longer within the oil, it suddenly becomes visible.