# If 2 cm thick glass blocks 40% of the light how much 1 cm thick blocks

• Jan 2nd 2013, 02:25 PM
ManIsuck
If 2 cm thick glass blocks 40% of the light how much 1 cm thick blocks
This is such a stupid problem, but my brain is completely locked when I try to figure out the rationale for why it is A or B. A it is 20%, because 1 cm is 50% of 2 cm and 20% is 50% of 40%. or B it is 1-0.6^0.5=0.22.. ~22% because the 2 cm thick lets 60% in and the 1 cm is half of it, so that type of equation would seem to make some sense.

Thanks for the help.
• Jan 2nd 2013, 03:23 PM
chiro
Re: If 2 cm thick glass blocks 40% of the light how much 1 cm thick blocks
Hey ManIsuck.

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.