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Different wavelengths of light can be refracted at slightly different angles as they pass through a material, and this is what causes dispersion, the effect seen when a beam of light passes through a prism and is split up into the different wavelengths of light.
The name Abbe comes from the German physicist Ernst Abbe, who defined the Abbe number.
Abbe controls the amount of dispersion. The higher the Abbe number, the less dispersion will be visible in the render. An Abbe number higher than 60-70 will render as if dispersion was not activated.
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