A positive lens has a focal length f.The only way to get a magnification of -1 is to
A)place a real object at the focal point.
B)place a real object at a distance 2f from the lens.
C)place a virtual object at a distance 2f from the lens.
D)Magnifications can never be negative.
E)None of these is correct.

Question

Quizplus · Accepted Answer

According to the lens formula, 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. For a positive lens, u is positive for a real object, and v is positive for a real image. 
When the magnification m = -1, we have v/u = -1. 
Substituting this in the lens formula, we get:
1/f = 1/v + m/u = 1/u - 1/u = 0
Therefore, u = infinity, or the object must be at the far point of the lens. 
For a positive lens, the far point is at a distance 2f. Therefore, the only way to get a magnification of -1 is to place a real object at a distance 2f from the lens.

A Positive Lens Has a Focal Length F