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Using the equation \( \frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2} \), determine the combined focal length.

1. \( \frac{1}{f} = 0 \)
2. \( \frac{1}{f} = \frac{1}{25} \)
3. \( \frac{1}{f} = \frac{1}{f_1} \)
4. \( \frac{1}{f} = \frac{1}{f_2} \)

Correct answer: \( \frac{1}{f} = \frac{1}{25} \)

Solution

The formula given is for the combination of two lenses. The power of the combination is the reciprocal of the focal length, and the correct interpretation is that the combined focal length is the sum of the reciprocals of the individual focal lengths.

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