Help please -- convex lens problem and focul length

[JayFish231]

Homework Statement

Suppose the convex lens is placed at the 49.4 cm mark, and an image forms on the screen at the 70.5 cm mark. Find:
A) The focal length of this lens: ____ cm
B) the absolute value of the magnification of this lens: ________
C) the height of the image: ________cm

Homework Equations

1/f = 1/d0 + 1/di

The Attempt at a Solution

I tried putting in 49.4 into d0 and 70.5 into di and trying to solve for focal length (f), the answer isn't right?? i don't know how to solve the remainder of this question...

 

[CivilSigma]

Hello Jay,
1. The lense is convex, so we know the focal length will be positive.
$$\frac{1}{s} + \frac{1}{s'} = \frac{1}{f} $$
$$f=(\frac{1}{49.4} + \frac{1}{70.5})^{-1}$$

2.Magnification for lenses is: $$ m = |\frac{-s'}{s}| = |\frac{y'}{y}|$$, you are given both s' and s.

3. From the above equation you can solve for y' , are you given the initial height of the obect?

Hope this helped.
-Sakon

 

[andrevdh]

Assuming the object is at zero the object distance is 49.4 cm, but the image distance is not 70.5 cm,
it is the distance from the lens to the image.