# Homework Help: (easy) Finding variables with mirror equation

1. Dec 19, 2012

### classi

This isn't exactly a homework problem, per se, but this was a district assessment I already took today. I was confident that I would get a good grade, but it turns out I got a measly 65%. I don't remember the exact wording, but I do remember the general idea of it.

1. The problem statement, all variables and given/known data
There are two lenses, both of which are 16cm apart. An object is 1.3cm away from the first lens.

First lens
f = 1.2cm
do = ?
di = ?
M = ?

Second lens
f = .42cm
do = ?
di = ?
M = ?

Total Magnification
Mtot = ?

2. Relevant equations
di = dof/do-f
M = di/do
Mtot = M1*M2

3. The attempt at a solution
First lens
f = 1.2cm
do = 1.3cm (given)
di = 15.6 cm
M = 12x

Second lens
f = .42
do = 16cm+1.3cm = 17.3cm
di = .43 cm
M = .025x

Total Magnification
Mtot = .3x

Looking back at it now, I think I messed up somewhere in the second lens.
I've asked friends what they got for Mtot, and the majority of them said a number around the 200s.

What am I doing wrong?

2. Dec 19, 2012

### SammyS

Staff Emeritus
Hello classi. Welcome to PF !

The image formed by the first lens becomes the object for the second lens.

3. Dec 19, 2012

### classi

Oh my gosh, thanks! :)
That makes perfect sense; I don't know why I didn't think of that before.

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