Hello,
I have a question from Princeton Review book which doesn't seem to have the right answer. Here it is:
If a certain eye can only focus on objects at least 50 cm away, which of the following lenses, if placed in front of the eye, would allow it to focus on an object 25cm away?
A. A converging lens with focal length 17cm
B. A diverging lens with focal length 17 cm
C. A diverging lens with focal length 25 cm
D. A diverging lens with focal length 50 cm.
The answer in the book is D.
I do agree that the focal length of the lens should be 50 cm, but shouldn't the lens be CONVERGING since it is decreasing the focal point of the eye?
Also, I had a more general question about lenses: I understand that lenses correct myopic and hyperopic sight via changing the image distance, whether closer to the lens (for myopic) or further from the lens (hyperopic) which then becomes the "object" for our eyes, but when I look through my diverging specs the objects seem further than closer. Based on the lens equation (1/o+1/i=1/f) the image distance should be lower than the object's distance.
Thank you for your input.
I have a question from Princeton Review book which doesn't seem to have the right answer. Here it is:
If a certain eye can only focus on objects at least 50 cm away, which of the following lenses, if placed in front of the eye, would allow it to focus on an object 25cm away?
A. A converging lens with focal length 17cm
B. A diverging lens with focal length 17 cm
C. A diverging lens with focal length 25 cm
D. A diverging lens with focal length 50 cm.
The answer in the book is D.
I do agree that the focal length of the lens should be 50 cm, but shouldn't the lens be CONVERGING since it is decreasing the focal point of the eye?
Also, I had a more general question about lenses: I understand that lenses correct myopic and hyperopic sight via changing the image distance, whether closer to the lens (for myopic) or further from the lens (hyperopic) which then becomes the "object" for our eyes, but when I look through my diverging specs the objects seem further than closer. Based on the lens equation (1/o+1/i=1/f) the image distance should be lower than the object's distance.
Thank you for your input.
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Yeah I would agree it should be a converging lens. Based on the fact that the eye can only focus on objects at 50cm or more, the guy has farsightedness, which is corrected using converging lenses. I'm not sure why the diverging lens is the answer, unless the question actually goes into deeper quantitative reasoning.
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be very careful about negative values of 1/d and the " virtual" images
a scale dreqwing is always a good way to check the algebra
I think the idea of seeing an object inside the f is to make the rays appear as if they came from further away
in any case you have a TWO lens system, eyeglass and eye lens
a scale dreqwing is always a good way to check the algebra
I think the idea of seeing an object inside the f is to make the rays appear as if they came from further away
in any case you have a TWO lens system, eyeglass and eye lens