How is light reflected and refractedwithin a natural tooth?

Human enamel contains approx.. 97% by weight mineral matter, mostly in the form of hydroxyapatite. The hydroxyapatite crystals in enamel are extremely small
1. Hydroxyapatite crystals align in organized, tightly packed masses to form prismatic enamel rods. Refraction will occur as light passes through each enamel rod and also at the internal (eg, cracks) and external surfaces of the tooth. An example of an internal surface within a porcelain-bonded restoration is the interface between a pressed coping and the superficial veneering porcelain.
2. The enamel rods or prisms are bound together with organic material (the collagen matrix) which represents approximately 1% of total mass.
  1. Enamel is therefore very translucent and may transmit up to 70% light through a 1mm thick section. By contrast the dentine only contains about 70% hydroxyapatite and the apatite crystals are much smaller than in enamel. Consequently they have an even greater surface area/volume ratio, it is still translucent but will generally not transmit much more than 30% light.
Colour
1. When light shines on your teeth, it is scattered and reflected in the enamel, creating the colour of your teeth. If you have plaque on your enamel, your enamel will reflect less light and look darker. If your dentin (the layer under your enamel) is darker, it may show through the enamel and create a darker appearance.
2. The total colour effect is derived from a combination of light directly reflected from the tooth surface combined with the light that has been reflected from the dentine which has already undergone some internal reflection.
3. The dentine is the prime source of colour and the reflected light emitted via the enamel are modified by the thickness and degree of translucency of the enamel and the dark oral background.
Translucency
1. Translucency in human enamel is influenced by the reflection and refraction of light in enamel rods.
  1. as the enamel approaches the tip of the incisal edge, the amount of light transmitted will increase to a degree where it almost becomes transparent.
2. Translucent enamel displays the characteristic of opalescence. Opalescence causes tooth enamel to reflect blue light back to the observer. Blue light tends to bend/refract more or to scatter within the enamel body. Longer red-yellow wavelengths do not bend as much in the enamel;
Chroma
1. chroma of a natural tooth comes mainly from the dentin, and the thickness and opacity of the overlying enamel determines how much chromatic influence the dentin has. When the enamel is thin at the gingival third but thick incisally, a chroma gradient is created. Increasing opacity of the enamel, as seen with dehydration and bleaching, can exaggerate the chroma gradient.
When light enters a natural tooth it gets bounced around the enamel like a fiber-optic cable. If one side of a tooth is illuminated with a curing light, the entire crown is lit. When light travels from one translucent material to another, light will be either reflected at the surface, or it will pass through the surface but bends (refracts) as it passes.

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How is light reflected and refractedwithin a natural tooth?

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