# Homework Help: To determine the focal length , f and position, d, of a converging lens mounted in

1. Dec 31, 2012

### lionely

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

I have to find f and d
Here's the method of the lab
Method: The length of the opening in the ray box was measured and
recorded. The equipment was set up as shown in the diagram.
The distance, x, of the tube from the screen was varied and
recorded. The magnification, m, for each value of x was
determined and recorded; this was obtained by focusing
the image of the cross-wire formed on the screen and
measuring the length of the image using the graph
paper attached to the screen. At least 8 values of x
and m were determined. A graph of m against x
was plot and a best straight line was drawn through
the points. Theory suggested that

m = (x + d) - 1
f
By writing the equation above in the form y=mx + c,
the focal length of the lens and the position of the lens
from end A of the cardboard tube was determined.

2. Relevant equations
m = (x + d) - 1
f
3. The attempt at a solution

I plotted a graph of m against x gradient = m/x

so gradient would = 1/f right?

then i could solve for d after?

Last edited: Dec 31, 2012
2. Dec 31, 2012

### TSny

Re: To determine the focal length , f and position, d, of a converging lens mounted

Hi lionely. Can you post the diagram?

3. Dec 31, 2012

### lionely

Re: To determine the focal length , f and position, d, of a converging lens mounted

it's there now

4. Dec 31, 2012

### TSny

Re: To determine the focal length , f and position, d, of a converging lens mounted

Just to make sure, is this the equation?

$m = \frac{x+d}{f} -1$
Yes, the gradient (slope) would be 1/f. The y-intercept of your graph is related to d. So, you could use the y-intercept to find d.

5. Dec 31, 2012

### lionely

Re: To determine the focal length , f and position, d, of a converging lens mounted

Okay thank you!

6. Dec 31, 2012

### lionely

Re: To determine the focal length , f and position, d, of a converging lens mounted

I get a negative value when i try to work out d... I think something is wrong.

7. Dec 31, 2012

### lionely

Re: To determine the focal length , f and position, d, of a converging lens mounted

Nevermind I made a mistake ....