# Homework Help: Help please -- convex lens problem and focul length

1. Apr 27, 2015

### JayFish231

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

2. Relevant equations
1/f = 1/d0 + 1/di

3. 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 isnt right?? i dont know how to solve the remainder of this question...

Last edited by a moderator: Apr 27, 2015
2. Apr 27, 2015

### 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

3. Apr 28, 2015

### 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.