log/expo question (i think) (1 Viewer)

scarvesss · May 12, 2012

a layer of plastic cuts out 15% of the light and lets through the remaining 85%

ii. how many layers of the plastic are required to cut out at least 90% of the light?

this seems easy but i think im just missing something

nightweaver066 · May 12, 2012

Sounds more like series and sequences and then log & exponential

$Let the intensity of light be given by $ P_n $ where n is the number of layers of plastic$

$P_0 = I$

$P_1 = I(0.85)$

$P_2 = (I(0.85))(0.85)$

$= I(0.85)^2$

$\therefore P_n = I(0.85)^{n}$

$\frac{P_n}{P_1} = \frac{I(0.85)^{n}}{I}$

$= (0.85)^{n}$

$To cut out at least 90 percent of the light, 10 percent or less light must remain$

$\therefore (0.85)^{n} \leq 0.10$

$(n)ln(0.85) \leq ln(0.1)$

$n \geq \frac{ln(0.1)}{ln(0.85)}$

$n \geq 14.168$

$\therefore n = 15$

scarvesss · May 12, 2012

wow nice, thanks man

log/expo question (i think) (1 Viewer)

scarvesss

howdy doo

nightweaver066

Well-Known Member

scarvesss

howdy doo

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