# Homework Help: Lens and mirror question, can't understand solution

1. May 11, 2014

### angelelizabeth

1. The problem statement, all variables and given/known data
A lens of focal length 0.8m is 1.0 m to the left of a mirror of focal length -0.500m. An object is placed 1.00 m to the left of the lens. Light goes through the lens, bounces from the mirror, and goes through the lens again, this time from the right.

2. Relevant equations

1/f=1/di+1/do

3. The attempt at a solution

(THIS IS FROM THE OFFICIAL SOLUTION MANUAL, I AM CONFUSED ON PART OF THE ANSWER TOWARDS THE END)
1/di1=1/f1-1/do1
1/di1=1/0.8-1/1
di1=4.0 m

This image is 3 meters behind the mirror, so do2 for the mirror is -3.0 m.

1/di2=1/f2-1/do2
1/-0.5-1/-3=1/di2
di2= -0.6 m

This image is behind the mirror.

THIS IS WHERE I GET CONFUSED:
di3 (for the lens)=1+0.6m=1.6m

1/di2=1/f1-1/do3=1/0.8-1/1.6
di3=1.6m

In this instance, why is di3 postive 1.6 m instead of negative 1.6 meters? The "object" is to the right of the lens, so shouldn't di3 be negative?

2. May 12, 2014

### dauto

If light is moving from left to right than left is positive and right is negative (that's the convention for an object), but if light is moving from right to left (and that's the case here) than right is positive and left is negative.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted