Question

what is focal length ???​

Answer 1

The distance between the pole and the focus is called the focal length. It is represented by f.

Answer 2

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ᴡʜᴀᴛ ɪs ғᴏᴄᴀʟ ʟᴇɴɢᴛʜ?

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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.