Refraction is what occurs when a light (such as an electromagnetic light wave) enters a new medium (type of matter- such as solid, liquid, or gas) that causes its speed to change. For example, when light passes from a fast medium (gas) to a slower medium (such as a liquid), then the light ray will bend toward the normal to the boundary between the two medium.

Refraction is what accounts for the "bending of a straw" when it is placed in a cup of water. It is also what causes a spear-fisherman to have difficulty in pinpointing the exact location of the fish that he/she intends to spear. Finally, a prism works based on the principles of refraction. Long wavelengths (red) refract less than short (blue or violet), so the colors of visible light separate.

the bending of light when traveling from one medium to another medium is known as refraction.

the fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density.

change in direction of propagation of any wave as a result of its travelling at different speeds at different points along the wave front.

measurement of the focusing characteristics of an eye or eyes.