The refractive index of a medium may be defined as the ratio of the speed of light in air (or vacuum) to the speed of light in the medium. Thus,
Refractive index = 
The refractive index of a medium has no units and it is represented by the letter ‘n’. In the table the value of refractive index of some important media is given.
Table of Different Values Refractive Index of Different Media
| S No. | Name of the material medium | Refractive index value |
| 1. | Diamond | 2.42 |
| 2. | Rock salt | 1.54 |
| 3. | Glass | 1.52 |
| 4. | Water | 1.33 |
| 5. | Ice | 1.31 |
| 6. | Air | 1.00 |
Use of Refractive Index
Actually the refractive index of a medium gives us the information about the capability of that medium to bend the light rays. For example, from the table it is clear that the value of refractive index of diamond is more than glass. So, the light rays will bend more on passing from air to diamond then from air to glass.
Test Your Understanding and Answer These Questions:
- Define refractive index. What is its S.I. unit?
- What is the value of refractive index of diamond?
- What is the refractive index of air, water, glass and ice?
- What is use of refractive index?
- Light enters from air to glass plate having refractive index 1.50. What is the speed of light in the glass? The Speed of light in vacuum is 3 × 108 m/s.