# 2 focal points, find image distance.

#### rcmango

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

An object is placed in front of a converging lens in such a position that the lens (f = 15.0 cm) creates a real image located 21.0 cm from the lens. Then, with the object remaining in place, the lens is replaced with another converging lens (f = 16.0 cm). A new, real image is formed. What is the image distance of this new image?

this is familiar with what i've been working with, but this time there is only one answer and two focal points, so no need to use one focal point, but both! I was unsure how to use both focal points for one answer.

2. Relevant equations

http://img246.imageshack.us/img246/6578/60992169pe8.png [Broken]

3. The attempt at a solution

not sure what i'm solving for, i believe i solve for u, can anyone show the setup for this one please. what i'm solving for exactly?

Last edited by a moderator:

#### Doc Al

Mentor
this is familiar with what i've been working with, but this time there is only one answer and two focal points, so no need to use one focal point, but both! I was unsure how to use both focal points for one answer.
Use them one at a time, as you have two different lenses. Use the data from the first lens to find the object distance. Then use that object distance with the second lens to find the image distance.

#### yosite

hopefully my answer is right... so 1st condition, we can see its real image, so its positive in magnitude. s1 is the distance from image to lense.. s1=+21 and f=15... with the formula.. we can find the s0 (distance object to lens) is 52.5 cm.... next for 2nd condition, f=16 cm.. put s0 and f on the formula to find s1.. u can get the distance of the image is 23 cm from the second lens.

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