For an object at infinity (s=oo). say a near-sighted person has an image form 2 cm behind their eye's lens, but the problem is that this is 0.5 cm short of their screen (retina). What is the focal length f (in cm) of a corrective lens that makes the image appear perfectly 2.5 cm behind the lens? (Treat the problem as the ideal case of two thin lenses right next to each other. Hint: It is kind of a bad name for it, but "focal length f" can be positive or negative, and the sign matters.) Same as the previous problem (lens to retina 2.5 cm, object at s=oo), except for a far-sighted person where the uncorrected image forms 0.5 cm behind their screen (retina). Now what is the focal length f (in cm) of a corrective lens that makes the image form perfectly on the retina 2.5 cm behind the lens?

For an object at infinity (s=oo). say a near-sighted person has an image form 2 cm behind their eye's lens, but the problem is that this is 0.5 cm short of their screen (retina). What is the focal length f (in cm) of a corrective lens that makes the image appear perfectly 2.5 cm behind the lens? (Treat the problem as the ideal case of two thin lenses right next to each other. Hint: It is kind of a bad name for it, but "focal length f" can be positive or negative, and the sign matters.) Same as the previous problem (lens to retina 2.5 cm, object at s=oo), except for a far-sighted person where the uncorrected image forms 0.5 cm behind their screen (retina). Now what is the focal length f (in cm) of a corrective lens that makes the image form perfectly on the retina 2.5 cm behind the lens?