What Does Index of Refraction Mean?

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Light does not always travel at a constant speed. It’s velocity is 300 million meters per second (over 186,000 miles per second) in a vacuum. The reason an object in a glass of water looks bent is because light travels at different speeds depending on what it goes through. More technically, as light passes through various media, it’s velocity will change. There’s a handy number that indicates how light will bend: this is called the index of refraction, and it is a ratio of the speed of light in a vacuum to the speed of light in a medium.

Light is energy?

Light is energy, and in a way, it’s something else. It can be described in terms of frequency, wavelength, speed, and energy. As its wavelength decreases, its energy and frequency increase. As its wavelength increases, its energy and frequency decreases. Since light’s speed is constant, its frequency stays the same when it passes through a material. This means that its wavelength must decrease for the speed to decrease.

Different “types” of light have different energies. Sometimes light particles will hit atoms in a medium and its energy will transfer into the atom. The atom then releases this energy and the light travels as it was before. This delay, though very short, gives the impression that light is bending.

When we see a straw in a glass of water, it appears as if it’s bent. This is an illusion caused by the light hitting the straw going  from air through water, glass, then air again. Air, water, and glass have different indices of refraction and thus the light “bends” three times before hitting our eyes.

Particles faster than light?

Yes, it’s possible! The phenomenon dubbed Cherenkov radiation occurs when electrons travel faster than light through a medium, yet slower than light in a vacuum. The electron decays from nuclear reactions and breaks a sort of light barrier (like a sound barrier). It gives off a photonic boom (like a sonic boom) in which light energy is strongly emitted. Some of this energy is emitted as visible light. Since blue light has such a short wavelength, it is more easily dispersed and is the dominant color we see.

Negative indices of refraction?

Earlier, I defined the index of refraction as the ratio of the speed of light in a vacuum (c) to its speed in a medium (v). This is actually a very simplified definition and is not completely correct. Compare these two equations:

n= \frac{c}{v}

n= 1+\frac{Nq^2_e}{2\epsilon_0m(\omega_0^2-\omega^2)}

Note that the second equation is specific to the index of gases, although the index for solids is even more complex. 

Don’t fret too much about all of the variables in the second equation. We are going to only focus on two: ω (omega) and ω(omega zero, or omega naught).

Every medium has what is called a resonant frequency (ω0). When light passes through a medium at this frequency, it will pass as quickly through it as it would through a vacuum. There is no medium besides a vacuum for which this is true, however air has a very close index of refraction (1.00029). This equation also shows us that the index for a material is not the same for every type of light; it varies with the light’s frequency. If the frequency of light, ω, is sufficiently higher than the resonant frequency of the material, a negative index of refraction is possible. What does this mean?

It means light can bend backwards! If the light has a higher frequency than the medium’s resonant frequency, the index can become negative. I say “can” because it is quite difficult to produce a material in which a negative index can be produce. Materials like this do not occur naturally, but are specially produced. These negative index materials are in a separate class of materials, called metamaterials. They act in ways which are completely different from conventional materials. For a closer look on negative index materials, click here to see how permittivity and permeability play a role.

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