AAMC FL 1 C/P Question 45

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AAMC FL 1 C/P Question 45

Post by ddurham » Mon Apr 15, 2019 1:58 pm

Can someone please help me on this lens equation. Obviously, my math is off somewhere. Thank you.
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Re: AAMC FL 1 C/P Question 45

Post by NS_Tutor_Will » Mon Apr 15, 2019 4:29 pm

Thanks for the question!

Q45 is definitely a tricky one since they don't give you numbers and are asking for a ratio!

Copying the below from an old post (viewtopic.php?f=16&t=317&p=1355&hilit=f ... 86d0#p1355):
This question requires us to use the thin lens formula and magnification.
Starting with what's given to us, we can see that the object distance is 3 times the focal length, or in other words o=3f.

The thin lens formula is 1/f = 1/o + 1/i. From here we can plug in our numbers, move things around and find out what i is in terms of f.
1/f = 1/3f + 1/i
3/3f -1/3f = 1/i
2/3f = 1/i
i = 3/2f

Now let's go back to question stem, it asks for the ratio of the height of the image to the height of the object, this is essentially magnification. i is 3/2f, o is 1/3f, so our ratio is 1/2.
I hope this helps! Feel free to ask any clarifying questions if something isn't clear!
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