# Homework Help: Lens problem

1. Sep 29, 2008

### thompson.1674

1. The problem statement, all variables and given/known data
A student is reading a lecture written on a blackboard. The lenses in her eyes have a refractive power of 61.20 diopters, and the lens-to-retina distance is 1.643 cm.
(a) How far (in meters) is the blackboard from her eyes?
(b) If the writing on the blackboard is 6.00 cm high, what is the size of the image on her retina (including the proper algebraic sign)? (in cm)

2. Relevant equations
p=1/f
1/f=(1/do)+(1/di)
m=hi/ho=-di/do

3. The attempt at a solution
(a) 61.20=1/f
f=.0163 m
1/.0163=(1/.01643m)+(1/do)
do=2.06 m
(b) m=-1.643cm/206cm= -.00798 cm x 6cm= -.0479cm.

I am not really sure what I did wrong on these two problems?? Any help would be appreciated

2. Sep 29, 2008

### LowlyPion

For one thing part a) looks like the math is a little off.
1/f = 61.20
1/.01643 = 60.8643
difference = .3357 That suggests do = 2.98 m

3. Sep 29, 2008

### thompson.1674

k cool thanks i got it now