Index of Refraction of Light

Light usually travels in straight lines. In a vacuum, light travels at a speed of $3.00 \times {10}^8$ meters per second. Once light passes from a vacuum into any other medium, it slows down. The speed of light in the new medium depends on the material of the new medium.

Some media, such as air, allow light to pass through almost as fast as it would through a vacuum. In fact, air slows the speed of light only by about three ten-thousandths of one percent (0.0003%). Other media cause light to slow down much more. For instance, the speed of light in water and in glass slows to $2.25 \times {10}^8$ meters per second and $2.00 \times {10}^8$ meters per second respectively.

Figure 5 Light rays slow and bend as they pass through the curved glass lens. In this case, the result is a magnified image.