# Focal Length Problem

1. Nov 16, 2014

### Chase11

1. The problem statement, all variables and given/known data
A lens made of glass has one concave side with a radius of curvature of 5 cm, and one convex side with a raduis of curvature of 7.5 cm. What is the focal length of the lens? Is this a converging or diverging lens?

2. Relevant equations
nglass=1.5
nair=1.0
(1/f)=(n-1)[(1/R1)+(1/R2)
R1=5 cm
R2=7.5 cm

3. The attempt at a solution
1/f=(1.5-1)[(1/5cm)+(1/7.5cm)] = .5(7.5cm+5cm/.375cm) = 16.667
So f=1/16.667=.06cm

The answer is supposed to be -30 cm and diverging. I have done a lot of similar problems right, I don't understand what I keep doing wrong on this one.

2. Nov 16, 2014

### Simon Bridge

Check the sign convention.

3. Nov 16, 2014

### Chase11

I thought it had something to do with that, but I honestly don't know where to change the sign. Is R2 supposed to be -7.5 because it is convex?

4. Nov 16, 2014

### Chase11

I'm getting 30 cm now, but this answer is supposed to be negative. Why would my answer come out positive?

5. Nov 16, 2014

### ehild

Concave means negative radius and convex means positive one.

6. Nov 17, 2014

### Simon Bridge

... impossible to say without you show your calculation.
You probably misplaced a minus sign...