Final answer:
Magnification underwater is caused by the refraction of light, which is due to the different refractive properties of air and water that cause light rays to bend and make objects appear closer to the surface than they really are.
Step-by-step explanation:
The magnification of visual images underwater is primarily caused by the different refractive properties of air and water. When light rays enter or pass through a different medium, they change direction in a process known as refraction. This bending of light occurs because light waves travel at different speeds in different media, which is directly influenced by the substances’ refractive indices. In the case of looking at an object underwater from above, light rays bend towards the normal when they pass from water to air at the interface, making the object appear closer to the surface than it actually is—resulting in what is perceived as a magnification effect.