Question

define --
Focus & focal length ?​

Answer 1

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The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light.

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Answer 2

Answer:

An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The distance in air from the lens or mirror's principal plane to the focus is called the focal length.

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