To find the focal length of a convex lens, you can perform an activity called the lens formula method. This activity involves setting up a simple experimental setup and measuring the distances involved to calculate the focal length of the lens. Here is a step-by-step guide on how to perform this activity:

Materials needed:
- Convex lens
- Screen
- Candle
- Meter scale
- Stand

Experimental Setup:
1. Place a stand on a table and fix a screen on it.
2. Take a convex lens and mount it on the stand, ensuring that it is perpendicular to the table surface.
3. Place a candle on the table and light it. The flame of the candle should be at the same height as the lens.

Procedure:
1. Start by placing the screen at a distance of approximately 1 meter from the lens.
2. Adjust the position of the screen until a clear, inverted image of the candle flame is formed on it.
3. Measure the distance between the lens and the screen. Let's call this distance "v" (object distance).
4. Now, measure the distance between the lens and the candle flame. Let's call this distance "u" (image distance).
5. Record the values of "u" and "v" in meters.

Calculations:
1. Using the lens formula: 1/f = 1/v - 1/u, where "f" is the focal length of the lens.
2. Substitute the values of "u" and "v" into the lens formula equation.
3. Solve the equation to find the value of "f" (focal length).

Safety Precautions:
1. Handle the lens and candle with care to avoid any accidents or injuries.
2. Ensure that the flame of the candle is not too close to the lens to prevent any damage.

Conclusion:
By following this activity and calculating the focal length using the lens formula, you will be able to determine the focal length of the convex lens. This activity helps to understand the behavior of light rays passing through a convex lens and how they form images.

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