# Homework Help: Converging Lenses Concept.

1. Nov 17, 2013

### jvdamdshdt

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

1.Where would you place an object to obtain an image as far away from the lens as possible?

2. Suppose you have an image focused on a slide screen. You moved the image towards the lens. Which way would you move the lens to keep the image in focus? Explain

2. Relevant equations

None

3. The attempt at a solution

For 1 I am thinking far away from the image as possible would be at the focal length but do they mean a real image? That would be between the focal length and twice the focal length

For 2 I am thinking you would move the lens away to keep the image in focus.

2. Nov 17, 2013

### cepheid

Staff Emeritus
Are you familiar with the thin lens equation? It can be used to answer both of these questions.

For question 1: imagine you had parallel rays coming in "from infinity." They would be bent by the lens and all converge at the focal point on the other side. Now, reverse time (or equivalently, just reverse the direction of propagation). So, the origin of the rays becomes the destination, and vice versa. So, rays would diverge from the focal point, hit the lens, and be bent to be all parallel on the other side. So where would the image be in this situation? Is it possible to get farther away from the lens than that?

For question 2: I repeat: do you know the thin lens equation?

3. Nov 17, 2013

### jvdamdshdt

I do know the lens equation. 1/do + 1/di = 1/f . In question one the object at the focal length will produce an image at infinity. For question two I don't know if I am remotely close to getting the answer.

4. Nov 17, 2013

### cepheid

Staff Emeritus
For question 2, you are reducing di and keeping f the same (since the lens curvature doesn't change).