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Refractive index

Posted by Luxexcel on Mar 10, 2015 6:33:00 PM
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Light can travel from one material into another as long as that material does not fully absorb, or fully reflect the light. Most of the time it travels from air into another material, or vice versa.

For the explanation we will describe the effect of refraction and refractive index through the example of light travelling through air into another material, and then back out into the air again.

If the light travels perpendicular to the edge of the material, the direction of the light stays the same. If the light travels at another angle than perpendicular the path of the light will change.

The ray of light entering the material is called the incident ray, and the angle at which it hits the material is called angle of incident.

  • Light rays are bent when they pass at an angle in or out of materials such as glass, plastic or water. This effect is called refraction.
  • Light passing into an optically denser medium is bent towards the normal; light passing into an optically less dense medium is bent away from the normal.
  • Materials such as glass, water and paraffin are said to be optically denser than air.

The refractive index is a number for how much the light is refracted, or how much the beam is diverted. Air is determined to have a refractive index of 1,00. The larger the difference between the refractive index of air compared to the refractive index of the material the more the angle of the light changes. Most of the time the refractive index is higher than the refractive index of air, which means the material is optically denser than air. This is the case for all fluids, and all solid materials. Some gasses have a lower refractive index than air. The refractive index of a material is mostly indicated with one number, the refractive index at 589nm, which is an average wavelength within the visible light spectrum.

Refractive index      
  Average refractive index @ wavelength Abbe number
PMMA 1,49 589 58
PC 1,58 589 28
Glass BK7 (crown) 1,52 589 64
Glass BAF10 (flint) 1,67 589 47
LUX Opticlear 1,53 589 45
LUX Standard 1,53 589 41
Air 1,00 589 89
SiO 1,87 1860  
Water 1,33 589 56

Materials have different refractive indices at different wavelengths, the refractive index of a material usually follows a curve. Please see example of our LUX Standard, and LUX Opticlear material.

Reflective_index_comparison

In the graph above you can see that the refractive wavelength is different at different wavelengths. This is very common in transparent (optical) materials. Curious on the refractive index of 3D printed optics, please download our graphs. 

The different wavelengths of the visible light represent different colors, as shown in the picture shown . Each wavelength has its own refractive index.

Wavelenghts

The difference in refractive index over wavelength in light causes nice visible effects of white light being separated into different colors. This is called chromatic dispersion.

For most optical solutions people look for optics with a low dispersion, but for some applications people look for high dispersion to deliberately separate the colors.

Reflection instead of refraction

When the incident angle is at a such a low angle, the light does not enter the material, but reflects of the surface. This is when the light hits the material at an angle which is lower than the critical angle. This effect is indicated in the image below between water and air.

This kind of reflection is also actively used in optics designed today.

TIR (Total Internal Reflection) optics rely on this effect

Also RXI optics (invented by LPI in Spain) rely on this effect, counting on three internal reflections

 

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