The near point of a naked eye is 25 cm. When placed at the near point and viewed

Question

The near point of a naked eye is 25 cm. When placed at the near point and viewed by the naked eye, a tiny object would have an angular size of 5.2 10-5 rad. When viewed through a compound microscope, however, it has an angular size of -7.00 10-3 rad. (The minus sign indicates that the image produced by the microscope is inverted.) The objective of the microscope has a focal length of 2.0 cm, and the distance between the objective and the eyepiece is 16 cm. Find the focal length of the eyepiece in cm.


I tried following an example but still got it wrong.. would like an explanation thanks

Explanation / Answer

For small angles the angular size in radians is given by the object height divided by the distance from the eye. Thus, when an object has the greatest angular size and is located the farthest away, it must have the greatest height. This means that object B has the greatest height. Objects A and C have the same angular size, but object C is twice as far away, which means that object C must have a greater height than object A. The ranking in descending order is, then, B, C, A.

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