spread out. These light beams are physical analogues of light rays.
Refraction
Change in wave direction at interface
Caused by change in speed of wave32.2 - Index of refraction
Index of refraction of a material: The speed of
light in a vacuum divided by the speed of light
in the material.
Light travels at different speeds in different materials. The index of refraction of a
material provides a measure of the speed of light in that material. The symbol n
represents the index of refraction. The equation used to calculate the index of refraction
for a material is shown to the right. The table to the right shows the indices for some
common materials.
In the equation for the index of refraction, the speed of light in a vacuum is in the
numerator. Light travels at its maximum speed in a vacuum, so the index of refraction
for other materials is always greater than one.
The index of refraction depends on the wavelength of the light. The table in Equation 2
is for yellow light with a wavelength of 589 nm. The index depends on the wavelength
because the speed of light in a material depends on its wavelength. The speed of light
in any material is slower for shorter wavelengths, so the index of refraction is greater for
light of shorter wavelengths. For example, for a type of glass called crown glass,
n = 1.50 for red light, and n = 1.53 for violet light, which has a shorter wavelength
than red light.
The index of refraction of a material also depends somewhat on its temperature. For
certain crystalline materials, it also depends on the angle at which the light travels
through the crystal lattice. This is why the index of refraction for corundum is indicated
as approximate in the table.
The index of refraction for visible light in the Earth’s atmosphere is about 1.0003 at
standard temperature and pressure. Since we typically use two or three significant
figures, we treat that index as 1.00. At the same level of precision, the speed of light is
3.00×10^8 m/s.
Index of refraction
Speed in vacuum / speed in material