Experiment to Find the Focal Length and Power of a Diverging Lens.

P tot  = P c + P D         Equation 3

where P c  is the power of the converging lens, P D is the power of the diverging lens, and P tot is the power of the two lenses, converging and diverging, combined.

Apparatus

Lamp

Meter ruler

Screen

Object

Converging lens

Diverging lens

3 Lens holders (one for lens, one for screen and one for object)

Set up apparatus as follows:

Figure 2

Variables that effect Experiment:

Plan

Step 1: Measure the focal length and power of a converging lens:

1. Measure the focal length of the converging lens roughly by using light from a distant source and moving the lens and the viewing screen until a clearly focused image is obtained. The distance from the image to the lens is the same as the focal length, f.
2. Set up the apparatus as shown in figure 2. With the object to lens distance, u, at 6f , move the viewing screen until a clearly focused image can be seen.
3. Measure the image to lens distance and include uncertainty measurements (where the distance can be changed more but the image is still in focus).
4. Repeat procedures 2 and 3 for object to lens distance at 5f, 4f, 3f, 2f, and just over 1f.
5. Solve the lens equation:

(Where u is the object to lens distance and v is the image to lens distance) using the values found in the above procedures to give a more accurate focal length.

6. Use Equation 2 to find the power of the lens.

 

Step 2: Measure the focal length and power of the combination of the converging and diverging lens:

1. Measure the combined focal length of the diverging and the converging lens roughly by using light from a distant source and moving the attached lenses and the viewing screen until a clearly focused image is obtained. The distance from the image to the lenses is the same as the combined focal length, f.
2. Set up the apparatus as shown in figure 2 but with both the converging and diverging lenses in the lens holder. With the object to lens distance, u, at 6f , move the viewing screen until a clearly focused image can be seen.
3. Measure the image to lens distance and include uncertainty measurements (where the distance can be changed more but the image is still in focus).
4. Repeat procedures 2 and 3 for object to lens distance at 5f, 4f, 3f, 2f, and just over 1f.
5. Solve the lens equation (Equation 1) using the values found in the above procedures to give a more accurate combined focal length.
6. Use Equation 2 to find the power of the combined lens.

Step 3: Finding the focal length and power of the diverging lens:

1. To find the focal length of the diverging lens manipulate Equation 1 to give the equation:

And insert the value of the focal length found for the converging lens and for the combination of lenses.

2. To find the power of the diverging lens use Equation 2

and insert the value found in the step above for the focal length of the diverging lens.

 

Results

Converging lens:

Diverging lens: