The subject is at a distance of d = 1.8 meters from the lens. Determine the focal length of the lens if the image size is 5 times the size of the subject.
Object distance, "u=-1.8\\space m"
Image distance "=v"
Magnification, "m=5"
"m=\\dfrac{v}{u}"
"v=-5\\times 1.8=-9\\space m"
Lens formula,
"\\dfrac{1}{v}-\\dfrac{1}{u}=\\dfrac{1}{f}"
"\\dfrac{1}{f}=-\\dfrac{1}{9}+\\dfrac{1}{1.8}"
"f=2.25\\space m"
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