26.1

The Index of Refraction

As Section 24.3 discusses, light travels through a vacuum at a speed of . It can also travel through many materials, such as air, water, and glass. Atoms in the material absorb, reemit, and scatter the light, however. Therefore, light travels through the material at a speed that is less than c, the actual speed depending on the nature of the material. In general, we will see that the change in speed as a ray of light goes from one material to another causes the ray to deviate from its incident direction. This change in direction is called refraction. To describe the extent to which the speed of light in a material medium differs from that in a vacuum, we use a parameter called the index of refraction (or refractive index). The index of refraction is an important parameter because it appears in Snell’s law of refraction, which will be discussed in the next section. This law is the basis of all the phenomena discussed in this chapter.

| Definition of The Index of Refraction
| The index of refraction n of a material is the ratio of the speed c of light in a vacuum to the speed v of light in the material:

| (26.1)

Table 26-1 lists the refractive indices for some common substances. The values of n are greater than unity because the speed of light in a material medium is less than it is in a vacuum. For example, the index of refraction for diamond is n = 2.419, so the speed of light in diamond is . In contrast, the index of refraction for air (and also for other gases) is so close to unity that nair = 1 for most purposes. The index of refraction depends slightly on the wavelength of the light, and the values in Table 26-1 correspond to a wavelength of in a vacuum.

Table 26-1 Index of Refraction* for Various Substances

Substance | Index of Refraction, n
Solids at 20 °C |
Diamond | 2.419
Glass, crown | 1.523
Ice (0 °C) | 1.309
Sodium chloride | 1.544
Quartz |
Crystalline | 1.544
Fused | 1.458
Liquids at 20 °C |
Benzene | 1.501
Carbon disulfide | 1.632
Carbon tetrachloride | 1.461
Ethyl alcohol | 1.362
Water | 1.333
Gases at 0 °C, 1 atm |
Air | 1.000 293
Carbon dioxide | 1.000 45
Oxygen, O2 | 1.000 271
Hydrogen, H2 | 1.000 139

a) |  | Measured with light whose wavelength in a vacuum is 589 nm.

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