What's Wrong with Refractive Index Query? | Wikipedia

In summary, refractive index is given by n^2 = (Relative Permittivity) x (Relative Permeability). However, for water specifically, the relative permeability is approximately 1.0 and the relative permittivity is 80.4. This results in a refractive index of 1.77, which may seem to contradict the expected value of 1.33. However, this discrepancy is due to the fact that the electric permittivity of water varies with frequency and the values used for comparison are at different frequencies. An electric field will indeed refract when entering water, and this is true for electromagnetic waves at all frequencies, including radio waves, x-rays, and gamma-rays. The law of ref
  • #1
RichRobX
11
0
Wkipedia gives refractive index

n^2 = (Relative Permittivity) x (Relative Permeability)

but taking water as an example
n = 1.33
Relative Permeability = 1.0 (roughly)
Relative Permittivity = 80.4

1.33^2 = 1.77 Which is NOT 80.4


:grumpy: what's wrong?
 
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  • #3
Keep in mind that the electric permittivity varies with frequency. You are comparing the values for an optical frequency, compared to a very low (A few hundred Hz typically) frequency, hence the apparent discrepancy.

The true value of the electric permittivity at optical frequencies is reliably obtained by measuring the refractive index.

Claude.
 
  • #4
IIRC we still can't calculate water's refractive index right via electron structure calculations. Water truly is mysterious stuff.
 
  • #5
Electric fields

Ok thanks that's cleared things up somewhat.

Does this mean that an electric field would be refracted upon going from say air into water?
 
  • #6
An electromagnetic wave oscillating at optical frequencies (i.e. light) will refract, yes.

Claude.
 
  • #7
claude,

Would radio waves, x-rays and gamma-rays also refract when entering water (guess I'm assuming no absorbtion which i shouldn't, but still).
 
  • #8
The law of refraction is derived from conservation of energy and momentum constraints at a boundary between two media with different refractive indices. Nothing in the derivation explicitly forbids refraction above or below a certain wavelength.

These restrictions are placed by more 'practical' considerations such as absorption.

Claude.
 
  • #9
uhhhh so then yes.
 
  • #10
Indeed

Claude.
 

1. What is refractive index and why is it important?

Refractive index is a measure of how light bends as it passes through a material. It is an important property in optics and is used to determine the speed of light in different mediums. It also affects the appearance of objects, such as making them appear larger or smaller, and can be used to identify materials.

2. How is refractive index measured?

Refractive index is typically measured using a refractometer, which is an instrument that measures the angle of light as it passes through a material. The angle is then used to calculate the refractive index using Snell's law.

3. What factors can affect the accuracy of refractive index measurements?

The accuracy of refractive index measurements can be affected by several factors such as temperature, impurities in the material, and experimental errors. It is important to control these factors in order to obtain accurate and reliable results.

4. What are some common applications of refractive index?

Refractive index has many practical applications, including in the manufacturing of lenses, prisms, and other optical devices. It is also used in the pharmaceutical industry to identify and analyze drugs, and in the food industry to determine the quality and authenticity of products.

5. How does refractive index affect the appearance of objects?

Refractive index can cause objects to appear larger or smaller, depending on the difference in refractive index between the object and the surrounding medium. This effect is known as refraction and is the reason why objects underwater appear distorted or magnified.

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