# How thick should the glass be so that it will block 25% of the light entering the sunglasses?

#### fxacx

##### New member
The intensity, I of light in lumens, passing through the glass of a pair of sunglasses is given by the
equation I(x) = I0 (0.8)^x , where x is the thickness of the glass in millimetres and I0 is the intensity of
light entering the glasses. How thick should the glass be so that it will block 25% of the light entering
the sunglasses?

#### skeeter

##### Well-known member
MHB Math Helper
H
The intensity, I of light in lumens, passing through the glass of a pair of sunglasses is given by the
equation I(x) = I0 (0.8)^x , where x is the thickness of the glass in millimetres and I0 is the intensity of
light entering the glasses. How thick should the glass be so that it will block 25% of the light entering
the sunglasses?
$0.75 I_0 = I_0 (0.8)^x$

solve for x ...

#### HallsofIvy

##### Well-known member
MHB Math Helper
So the question is, again, do you know how to solve $a^x= y$? You titled this "Log/exponential word problem". Do you know what "logarithms" and "exponentials" are and how they are related?