Lens Object Screen Problem: Finding the Focal Length and Image Distance

[Saxby]

Homework Statement

An object is placed 2.4m from a screen, and a lens of focal length f is placed between the object and the screen so that a real image of the object is formed on the screen.

When the lens is moved 1.2m toward the screen, another real image of the object is formed on the screen

Homework Equations

1/s +1/s' = 1/f = 2/R

s = Distance between object and lense
s' = Distance between image and lense
f = Focal length
R = Radius of lense

The Attempt at a Solution

This question has me well and truly stuck so i apoligize if there isn't much of a solution but i'll do my best.

I understand if a real image is being projected then the image must be inverted when the lens is at both distances from the screen. I know the lens must be at a distance of less than 1.2m away from the object before it was moved.

I would usually try and draw a ray diagram but the nature of the lens (diverging or converging) is not stated.

Absolutely any help would be very much apprietiated :)

 

[voko]

For the first image, you have 1/s +1/s' = 1/f, and s + s' = D = 2.4 m.

For the second image, you have 1/(s + d) + 1/(s' - d) = 1/f, where d = 1.2 m.

These are three equations for three unknowns s, s' and f.

 

[Saxby]

Thank you, i understand how to get the answer now :)