Final answer:
Refraction is the phenomenon where light bends when passing from one transparent substance into another due to a change in speed. It is used to explain the functioning of lenses and the formation of images, and the index of refraction determines the degree of bending.
Step-by-step explanation:
The phenomenon where light changes direction as it passes from one transparent substance into another is known as refraction. This bending of light occurs because light waves change speed when moving between materials with different optical densities. For instance, light will refract differently when transitioning from air to water, water to glass, or glass to air.
Refraction is crucial in explaining how lenses form images and helps us understand various optical instruments and phenomena in our daily lives. The law of refraction, also known as Snell's law, describes how the path of light bends at the interface between two media. This law also involves the index of refraction, which indicates how much the light slows down in a given medium compared to vacuum and is always greater than or equal to 1.