This image displays refraction. Refraction is the bending of light when it speeds up or slows down after traveling through different materials. Here the medium that light travels through changes from air to water. Water has an index of refraction of 1.333- a higher index of refraction means that light will travel slower and have a greater change in direction. This phenomenon explains why the background looks different through the cup of water compared to when it's being seen through the air. In addition, the balloon has been filled with air, allowing it to float because air (1.225 kg/m3) is less dense than water (1027 kg/m3). Additionally, since the balloon is displacing some of the water inside the cup, there is buoyant force present and acting upwards. Here, the buoyant force is equal to the weight of water being displaced by the balloon and is also the same magnitude as the tension force, which is why the balloon floats. Atmospheric pressure makes sure that the index card doesn’t move and lets water come out. When the cup was flipped, pockets of air remained inside, and while some water leaked out, the air molecules inside were allowed to slightly expand and form an area of low pressure compared to the outside of the cup. Because pressure moves from areas of high to low concentration, there’s a force being applied on the index card opposite to the direction of the force caused by the weight of the water.