# Homework Help: Lens maker's equation used correctly?

1. Feb 9, 2012

### DannyPhysika

1. The problem statement, all variables and given/known data

An object is placed 60 cm from a thick double convex lens, with faces of radii of curvature 20 cm and 30 cm, with thickness of 2 cm, and index of refraction is 1.5. Find the position of the image. Ans: ( 39.9 cm)

2. Relevant equations

lens maker's equation, na/object + na/image = (nb - na)(1/R1 - 1/R2)

3. The attempt at a solution

I used this equation and got 40 cm, as opposed to 39.9. I figured it couldn't be a rounding error considering the numbers given. Am I supposed to use the 2 cm thickness somehow?

2. Feb 9, 2012

### utkarsh009

When I calculated the answer, it came out to be 40.9 cm. Strange!! I used the lensmaker equation given in Wikipedia. These were my results : f= 900/37 cm, u=-60 cm (given) So, using lens formula v = 40.9cm. And yes you need to use the thickness. See the lensmaker's equation in Wikipedia!!!

3. Feb 9, 2012

### utkarsh009

@Danny :the equation which you are using seems to be derived from thin lens equation where you neglect thickness as it's too small compared to Radii of curvature.