What is the focal length of the lens?

AI Thread Summary
To find the focal length of a diverging lens, the equation 1/di + 1/do = 1/f is used, where di is the image distance and do is the object distance. In this case, the image distance (di) is 4.1 cm below the lens, making it negative, while the object distance (do) is 12 cm. Substituting these values, the calculation shows that the focal length (f) is approximately 3.06 cm. The error in the initial calculation was not accounting for the virtual nature of the image, which requires di to be negative. Thus, the correct focal length of the lens is determined to be 3.06 cm.
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Homework Statement


When a diverging lens is held 12 cm above a line of print, as in Figure 26.29, the image is 4.1 cm beneath the lens. What is the focal length of the lens?
26-29fig.gif



Homework Equations


1/di + 1/do = 1/f


The Attempt at a Solution


1/di + 1/do = 1/f
1/4.1 + 1/12 = 1/f
f = 3.0559 cm

What did I do wrong?
 
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The image is virtual, so di is considered to be negative.
 

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