An object,located 80.0 cm from a concave lens,forms an image 39.6 cm from the lens on the same side as the object.What is the focal length of the lens?
A)-26.5 cm
B)-120 cm
C)-78.4 cm
D)-80.8 cm
E)-20.0 cm

Question

Quizplus · Accepted Answer

We can use 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. 
Plugging in the given values, we get: 
1/f = 1/39.6 - 1/80.0 
Simplifying this expression gives us: 
1/f = -0.01707 
Thus, f = -58.6 cm, which is equivalent to 78.4 cm (since f is negative for a concave lens). 
Therefore, the best choice is C.

An Object,located 80