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RefractiveIndex (RI)

• The refractive index of a transparent medium, involving light, indicates the degree to which a ray of light will be bent as it passes through that medium.
  
  
• That is, as light enters a transparent medium it is slowed down/up to a degree dependant on the material.
  
  This reduction/increase in speed causes the light entering at a angle to the surface of the new medium to change its angle of travel according to its refractive index.
  
  This occurs in an similar way, that a formation of soldiers entering a river on an angle, will alter their direction of travel (see diagram).
  
  
• The bending of light due to change in the refractive index of varying medium results in a number of physical phenomena -
  
  
  • A fish you see in a body of water is not actually located where you see it (see diagram).
    
    
  • On a hot summer's day, the rising hot air from a road can be observed as a shimmering appearance.
    
    This is because the heated air will have a different refractive index from that of the cooler surrounding air, resulting in an altered light direction through the hot air compared to that of the cooler air, giving rise to the shimmering appearance.
    
    
  • A diamond is pure carbon formed under great pressure, imparting it with a large refractive index.
    
    Light entering at an angle to the surface will be refracted/bent.
    
    As the angle of entry increases, it will reach a critical angle where the light will be internally reflected.
    
    This angle is smaller for materials with a higher refractive index and hence a larger quantity of the light entering the medium at a angle will be internally reflected.
    
    Diamonds and crystal glass (high lead content) have such high refractive indices and are faceted to a geometry that takes advantage of the internal reflection to return as much of the light entering the material back to the viewer, giving the piece in question a greater sparkle and lustre (see diagram).