This article introduces the topic of “refractive index” and “chromatic dispersion.”

Light slows down when it travels through materials, even if the material is transparent. That includes air. The refractive index, or index of refraction, is the number that specifies how much light slows down within a material. The higher the refractive index, the slower the light moves through that material. Refractive index is generally written as “n.” As stated in previous blogs, the speed of light in empty space is 300,000,000 meters/second, and it is indicated by the letter “c.”  Inside a material, the speed of light slows down to c/n. For example, in most, glass the refractive index is near n=1.5, so light travels only about 200,000,000 meters/second. That is very fast, but it is much slower than the speed of light in open space. The refractive index of air is around 1.0003 at sea level. That is very close to 1, but the difference can be significant for applications like astronomy, where light travels through miles and miles of air before it reaches the telescope.

Light rays bend when they move from one material to another. This behavior is called “refraction.”

Why Does Refractive Index Matter?

Refractive index is arguably the most important concept in optics. Almost every aspect of light’s behavior is affected by the refractive index of the material it is traveling in. The most significant impact of refractive index is that light changes direction when it moves between materials with different refractive indexes. That bending is called “refraction,” and the amount of bending depends on the difference in the refractive index between the two materials—that is how refractive index got its name. The photograph is by Matt Kuchta, associate professor at University of Washington–Stout, from his blog, Pascal’s Puppy.

Refraction is the subject of the next article.

 

A Complication: Chromatic Dispersion

Refractive index depends on the wavelength of the light.

A significant complication in optics is that the refractive index of a material is different depending on the color (wavelength) of light. This phenomenon is called “chromatic dispersion,” because “chroma” is the Greek word for “color.” In most cases the index of refraction decreases as the wavelength of light increases; in other words, the refractive index is usually higher for blue light than for red. The graph shows dispersion for one of the most commonly used glasses in the world: N-BK7 from the Schott glass company. For N-BK7 the refractive index drops from above 1.530 to below 1.513 as the light changes color from blue to red. That is only a 1.7% change, but the effects of dispersion are profound. More on that later.

From all of this information, we can create two “very least you need to know” definitions:

The Refractive index (or index of refraction), of a material is indicated by the letter “n,” and the index impacts almost every aspect of light’s behavior, including how fast light travels through a material and how much light bends when it moves from one material into another.

Chromatic dispersion causes the refractive index of a material to be different for different wavelengths of light.

 

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