Subject 2: Classifying materials by their properties.

• Classification in function of the material • Classification in function of Abbe number

Crown and Flint• Relation between the index of refraction

of a lens and its thickness (volume)• Relation between the index of refraction

of a material and its transmittance • Characteristics of the ideal material

Classification by function of material: mineral

•MINERAL MATERIAL:

•Manufactured by melting (1500 ºT) Si O2 and derivatives.

•Harder (more scratch resistant) and more fragile (less resistant to impact ) than organic material.

•Heavier (higher density) than organic material.

Classification by function of material: Organic

•ORGANIC MATERIAL:

•Manufactured from hydrocarbon derivatives at low temperatures.

•Softer (lower scratch resistance) and less fragile (impact resistant) than mineral material.

•Lighter (less density) than mineral material.

Classifying by function of Abbe Number

• Abbe number > 50 : material that doesn’t disperse much white light (desirable characteristic). Eg: Flint.

• Abbe number < 50 : material that disperses a lot of white light. Eg: Crown.

Relation of the index of refraction of a lens and its

thickness (volume)

• At a higher n, the surfaces are flatter. Maintaingin the dioptric power P and the diameter constant, the volume is less, which favors the esthetics of this lens.

• At a higher n, the density is greater, in a mineral lens.

Relation of a material’s index of refraction and transmittance

•As n increases, undesirable characteristics also increase:

• Higher reflection coefficient (less transparency).• Lower Abbe number, higher chromatic dispersion (colorful halos or color fringing).

Characteristics of the ideal material

•Unbreakable (impact resistant)

•Hard (scratch resistant)

•Light (low density)

•Thin(high index of refraction)

•Maximum transmittance

•Minimum dispersion (elevated Abbe number)