A small refracting telescope designed for individual use has an objective lens with a diameter of 6.00 cm and a focal length of 1.325 m. what is the f-number of this instrument?

F-number is 2.21 The f-number is defined as the ratio of the focal length to the diameter of the entrance pupil. So the equation is N = f/D where N = f-number f = focal length D = entrance pupil With the telescope, the entrance pupil will be assumed to be equal to the lens diameter. So substituting the known values into the equation. N = 1.325 m / 0.0600 m N = 2.208333333 Rounding to 3 significant figures gives a F-number of 2.21