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To calculate the focal length of a lens, use the lens formula: 1/f = 1/v - 1/u.

The focal length (f) of a lens is the distance from the lens to the point where it focuses parallel rays of light. To find this, you can use the lens formula, which relates the focal length (f) to the object distance (u) and the image distance (v). The formula is written as: 1/f = 1/v - 1/u. Here, 'u' is the distance from the object to the lens, and 'v' is the distance from the lens to the image.

To use this formula, you need to measure the object distance (u) and the image distance (v). Make sure to use the correct sign convention: for a converging lens, distances measured in the direction of the incoming light are positive, and those measured against the direction of the incoming light are negative. For a diverging lens, the focal length is considered negative.

For example, if you place an object 30 cm from a converging lens and the image forms 60 cm on the other side of the lens, you would substitute these values into the formula: 1/f = 1/60 - 1/30. Simplifying this, you get 1/f = (1 - 2)/60 = -1/60. Therefore, f = -60 cm. Since we are dealing with a converging lens, the focal length is positive, so f = 60 cm.

This method allows you to determine the focal length of any lens, provided you have accurate measurements of the object and image distances.

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