What is the primary reason an object appears larger underwater?

The primary reason an object appears larger underwater is due to the refraction of light. When light passes from one medium to another, such as air to water, it bends because of the change in speed of light in different materials. This bending of light, known as refraction, causes objects underwater to seem closer and larger than they actually are.

As light moves from water (with a higher density) to air (with a lower density), it changes direction, making it appear as though images are magnified. This optical illusion can significantly affect depth perception and the apparent size of objects while diving or viewing underwater scenes. Understanding this phenomenon is crucial for divers to accurately gauge distances and sizes of underwater objects.