How are the focal length of a lens and the distance from its main plane to a point source and its image obtained with this lens related?

bystudin | October 25, 2020 | Education

How are the focal length of a lens and the distance from its main plane to a point source and its image obtained with this lens related? Formulate the sign rules for the quantities included in this formula.

According to the formula: 1 / F = 1 / a + 1 / b
where F is the focal length of the lens; positive for a converging lens, negative for a diverging lens, a is the distance from the source to the main plane of the lens, b is the distance from the main plane of the lens to the image, taken with a “+” sign if the image is valid, and with a “-” sign if the image imaginary.

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