Focal Length Problem: -30cm Converging/Diverging Lens

[Chase11]

Homework Statement

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?

Homework Equations

n_glass=1.5
n_air=1.0
(1/f)=(n-1)[(1/R₁)+(1/R₂)
R₁=5 cm
R₂=7.5 cm

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.

 

[Simon Bridge]

Check the sign convention.

 

[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?

 

[Chase11]

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

 

[ehild]

Concave means negative radius and convex means positive one.

 

[Simon Bridge]

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

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