Material index formulas for optical clarity and Ashby charts

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Hello:)

i need some help trying to find some formulas that i can retrive the material index from at the end of simplification to use with an ashby chart to select materials. here is an example of deflection equation from a sphere used for a spherical cap and what material index we used to find our material.

Deflection:
d=(pr²)/2Et
If mass is equal to density multiplied by volume, we can derive an equation for thickness:
m=𝞺at
m=𝞺(2rh)t
t=m/𝝆2rh
d=((pr²)/2E)*m/𝝆a solve for m to find the material index
m=pr³h𝝆/dE
𝝆/E is our material index and should be minimized, or E/𝝆 should be maximized.

the last material index i need to find is for optical clarity, how well you can see through it. We are making a visor for a helmet. So it needs to be clear and non reflective, but i dont know how to find the material index because i do not know of any equations used for optical clarity.

thanks.
 

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