Question

what is focal length...​

Answer 1

Answer:

The distance between the convex lens or a concave mirror and the focal point of a lens or mirror is called the focal length. It is the point where parallel rays of light meet or converge.

Answer 2

Answer:

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Explanation:

...the distance between the centre of a lens or curved mirror and its focus.

the equivalent distance in a compound lens or telescope.

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.