There are two types of glass referred to in optics. Crown glass and Flint glass. Crown glass is the clearer of the two and is most commonly used. However, glass in general is the clearest material of all the mediums. In multi focal lenses (bifocal & trifocal) both crown and flint maybe used because of the different refraction scale allowing one lens type (segment) to be completely immersed within the other, giving a seamless surface. Crown glass is the clearest material with an Abbe Value of 59. Theoretically speaking, the higher the index of refraction the lower the Abbe Value will be.

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