# 2 focal points, find image distance.

• rcmango
In summary, the conversation discusses the use of two converging lenses and their focal points to determine the image distance of a real image formed by the second lens. The first lens (f=15.0 cm) creates an image located 21.0 cm from the lens, and using this data, the object distance (s0) is found to be 52.5 cm. The second lens (f=16.0 cm) then forms a new real image, and by plugging in the object distance (s0) from the first lens, the image distance (s1) is found to be 23.0 cm.
rcmango

## Homework Statement

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.

## Homework Equations

http://img246.imageshack.us/img246/6578/60992169pe8.png

## 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:
rcmango said:
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.

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.

## 1. What is a focal point?

A focal point is a point where light rays converge or appear to diverge after passing through a lens or reflecting off a curved surface. This is also known as the point of focus.

## 2. How is the image distance determined in a system with 2 focal points?

The image distance is determined by the distance between the object and the first focal point, divided by the distance between the object and the second focal point, multiplied by the distance between the second focal point and the image.

## 3. Can the image distance be negative in a system with 2 focal points?

Yes, the image distance can be negative if the image is formed on the same side of the lens as the object. This is known as a virtual image.

## 4. What changes can affect the image distance in a system with 2 focal points?

The image distance can be affected by changes in the distances between the object and the focal points, as well as changes in the refractive index of the medium the light is passing through.

## 5. How does the position of the object affect the image distance in a system with 2 focal points?

The position of the object can affect the image distance in a system with 2 focal points. Moving the object closer to one focal point will result in a larger image distance, while moving it closer to the other focal point will result in a smaller image distance.

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