Refraction occurs when light passes from one medium to another, causing a change in direction. Abrupt refraction is exemplified by a pencil appearing bent when placed in water, as the transition between air and water involves a sudden change in the index of refraction. In contrast, gradual refraction happens when light moves through mediums with varying densities, such as warm and cold air, leading to a more gradual bending of light. The key distinction is that abrupt refraction results from an immediate change in medium, while gradual refraction involves a more gradual transition. Understanding these concepts clarifies the differences between abrupt and gradual refraction.