L4(b) Optical Properties Of Lens Materials

Question 1

5 properties of lens materials

Answer 1

Refractive index, density, constringence, uv cut off, tensile strength

Question 2

Explain refractive index

Answer 2

• the higher the RI, the thinner the lens, lighter
• high index: powers greater than +/-4.00D
• formula: n(1)-1/n(2)-1

Question 3

lens materials to recommend for high refractive index

Answer 3

polyurethane, polycarbonate, trivex

Question 4

explain density

Answer 4

• the higher the ri, the higher the density, lens heavier
• glass lens naturally denser and hence heavier than plastic (by 2-4 times)
• trivex least dense

Question 5

lens materials for best density

Answer 5

trivex,pc, cr39, pu

Question 6

lens material for best UV cut off

Answer 6

pu, pc, trivex, cr39

• if px has low power hence using cr39, advice for uv coat
• glass lenses low uv cut off

Question 7

explain tensile strength

Answer 7

resistance of a lens against breakage when mounted into rimless frames

• 35kg above is good
• measured with instron

Question 8

lens material for best tensile strength

Answer 8

PU! pc tv cr39