It depends on how it blocks the light.
I'm going to assume that each individual slab blocks a specific percentage of light coming in.
In this case 2cm blocks 40% of the light which means 60% gets in.
This means that it blocks 1 - r - (1-r)r = 0.6 or 1 - r - r + r^2 = 0.6. I am blocking for the first 1cm slab and then I'm taking that result and blocking it again.
In this case we get 1 - 2r + r^2 = 0.6 so this leaves (1-r)^2 = 0.6 or 1-r = SQRT(0.6) or r = 1 - SQRT(0.6) since r < 1.