# Experiment: Measuring the focal length of a converging lens

• Taniaz
In summary: If you can't upload the video, it's difficult to see what you are doing in it. In summary, you are trying to set up a light source, an object grid, a converging lens and a screen so that a quantitative investigation of object and image distances can be performed. Tabulate corresponding object and image distances (p and p' respectively). Your values of 1/p, 1/p', p+p' and pp' are all different from the values that are given on the lens maker's packaging. The focal length that is given on the packaging is not the focal length that is actually achieved when the lens is used. The focal length that is achieved is closer to the value of 1/f
Taniaz

## Homework Statement

You are required to set up a light source, an object grid, a converging lens and a screen so that a quantitative investigation of object and image distances can be performed. Tabulate corresponding object and image distances (p and p' respectively). Also tabulate values of 1/p, 1/p', p+p' and pp'.

## Homework Equations

Theory suggests that 1/p + 1/p' = 1/f. Graph (i) p' versus p, (ii) 1/p' versus 1/p, (iii) pp' versus p+p'. Explain the shape of each graph. Determine its value in each case. Which method do you think its most accurate?

State, with reasons, the level of accuracy expected from each of your measurements and hence give a statement of how accurate you expect from each of your measurements and hence give a statement of how accurate you expect your value of f to be in each case.

## The Attempt at a Solution

I set up the experiment as shown in the pictures. I placed the object grid in front of the light source, followed by a plano convex lens (f=30 cm) followed by the screen. The problem with the object grid, I don't see an image of the object grid but of an inverted filament instead (not clear in the diagram). I then removed the object grid and I still saw an inverted image of the filament. Why am I not seeing the object grid? Everything is aligned. I only see the object grid very clearly when it's really close to the source after that it disappears and a very clear image of the filament appears.

Another issue is that when I measure the object distance on the meter ruler and and subtract it from the image distance which I also read from the meter ruler, I don't get the focal length actually mentioned on the lens pack. The focal length on the pack is 30 cm and I get a difference of 85 +/- 5.

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Try placing the object grid as close to the light source as you can; the grid is your object, and the source creates the rays you are trying to direct into your lens.

Hope this helps!

- Strelkov

Hi Strelkov,

Thanks for your reply. I tried doing that too, as I moved the lens further away, the object grid would turn into this blur circle of light and then suddenly, after further movement, a very clear, inverted image of the filament would show up. Quite strange.

Exactly!

Since you only have one lens, the resulting image will be inverted(recall ray trace diagrams). Once the image is clear and inverted, you have found the image; now use the lensmaker's formula to confirm your focal length.

- Strelkov

Yes I am aware that it will be inverted but my object is the grid and not the filament of the light source? And I don't get the focal length to be the same as what it said on its packaging?

Try this:
Position the lens where you get
Taniaz said:
...a very clear, inverted image of the filament...

Now move the light farther away from the lens and put the grid where the light used to be.

Ok I'll try that but why can't I seem to get it with the light source present?

I've attached a video of what I get. It's so clear.

Can't seem to upload it :/

Last edited:

## 1. How do you measure the focal length of a converging lens?

The focal length of a converging lens can be measured using the lens formula, which is 1/f = 1/u + 1/v, where f is the focal length, u is the distance between the object and the lens, and v is the distance between the lens and the image formed.

## 2. What materials are needed for this experiment?

To measure the focal length of a converging lens, you will need a converging lens, a light source, a screen or white paper, a ruler or measuring tape, and an object to serve as the source of light.

## 3. What is the principle behind this experiment?

The principle behind this experiment is based on the fact that when an object is placed at a certain distance from a converging lens, an inverted and magnified image is formed at a specific distance on the other side of the lens. By measuring these distances and using the lens formula, we can calculate the focal length of the lens.

## 4. Can this experiment be done with a diverging lens?

No, this experiment is specifically designed for measuring the focal length of a converging lens. The principle and formula used in this experiment are not applicable to a diverging lens.

## 5. How can errors be minimized in this experiment?

To minimize errors in this experiment, it is important to ensure that the object, lens, and screen are all aligned on the same axis. Additionally, using precise measuring tools and repeating the experiment multiple times can help to reduce any potential errors.

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