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A lens has a focal length of 28 $\mathrm{cm}$ and a diameter of 4.0 $\mathrm{cm} .$ What is the $f$ -number of the lens?
7
all right. So unless has focal length 28 centimeter and the diameter of the lens is for senator. And from here we want to find out the F number for the lens. So by definition, F number is the ratio between focal length and the diameter of the lens. So from here, you can see it is going to be 28 centimeter over four centimeter, which is going to be called to seven.