How Do You Calculate Focal Length in a Lens Lab Experiment?

[MandolinGirl]

In my physics lab, we have been assigned to find the focal length of a lense that was given to us. The professor says it can be found by using this formula: Object distance over 1 plus 1 over the Image Distance will equal one over the Focal Length.

So, we put a piece of white paper at ground level, then got a flashlight and measured its diameter and determined it to be 2 1/8. During our first attempt, the flashlight was held at a distance of 24” from the paper. The lens was held at a distance of 20” inches from the paper, and 4” from the light source. Then the amount of magnification was measured by measuring the light as it was projected through the lens and onto the paper. In the second attempt, we kept the light source at a constant distance of 24” from the paper, but changed the position of the lens by holding it 6” from the light source and 18” from the paper. We then measured the magnification of the light beam again and planned to use it for comparative purposes. In the case where the lense was held 4" from the light source, the magnification was 5 1/4 inches, and in the case where the lense was held 6" from the light source, the magnification was determined to be 4 1/2. I just don't understand how to use those numbers in a formula that will ultimately determine the focal length. Thank you for any help you can provide!

Sarah

 

[UrbanXrisis]

Object distance over 1 plus 1 over the Image Distance will equal one over the Focal Length.

I think it's 1/do+1/di=1/f

I remember it as "If I do I die" ----> 1/f=1/do+1/di

 

[M98Ranger]

, thank you for sharing your experience with the focal length lens lab. It sounds like you have made some progress in your measurements and observations. Let me try to help you understand how to use those numbers in the formula to determine the focal length of the lens.

First, let's review the formula given by your professor: Object distance over 1 plus 1 over the Image Distance equals one over the Focal Length. This formula is known as the thin lens equation and is used to calculate the focal length of a lens.

In your experiment, you have measured the object distance (the distance between the lens and the object, in this case the flashlight) and the image distance (the distance between the lens and the image, in this case the light beam on the paper). The magnification is the ratio of the image distance to the object distance.

To use the formula, you will need to plug in the values you have measured. Let's take the first attempt as an example. The object distance is 20 inches and the image distance is 4 inches. Plugging these values into the formula, we get:

20/1 + 1/4 = 1/Focal Length

Now, we can simplify this equation by finding the common denominator of 4. This gives us:

80/4 + 1/4 = 1/Focal Length

81/4 = 1/Focal Length

To isolate the focal length, we can take the reciprocal of both sides of the equation, giving us:

4/81 = Focal Length

So, in this first attempt, the focal length of the lens is 4/81 inches.

You can follow the same steps for the second attempt, using the values you have measured. Once you have calculated the focal length for both attempts, you can compare them to see if they are similar. This will help you verify the accuracy of your measurements.

I hope this helps you understand how to use the formula to determine the focal length of the lens. If you have any further questions, don't hesitate to ask your professor or a teaching assistant for assistance. Good luck with your lab!