In this video, someone uses the law of refraction. When light passes through air, the glass, and the water, the angle the light ray makes with the normal of the surface is changed every time it passes through the different medium. The glass, the air, and the water have different index due to their density relative to still air, so it causes the light to change speed which results in a change in direction. Because the change of direction is caused by a change in speed, whenever light passes from one medium into another, it refracts. The light comes towards the centre and gets closer to the normal, or vice versa. This results in the picture behind the jar becoming a reversed image.