Magnification of an Image formed by a converging lens

[fordy314]

Homework Statement

A converging lens with a focal length of 14.2 cm is used to inspect a coin. The lens forms a virtual image 30.1 cm away. What is the magnification?

Homework Equations

(1/Si) + (1/So) = (1/f)
M = (hi/ho) = (-Si/So)

The Attempt at a Solution

1/So = 1/f - 1/Si

1/(1/So) = So = 26.88cm

M = (-Si/So) = -30.1/26.88 = 1.12


I'm pretty sure that I have the solution and answer correct, but I just wanted to double check. This homework is online and the site is telling me that I have the answer wrong and I just wanted to make sure that I wasn't missing anything before I bother my teacher about it. As of right now I think it's just an error in the code of the site, but I'm just double checking. Thanks.

 

[TSny]

Homework Statement

A converging lens with a focal length of 14.2 cm is used to inspect a coin. The lens forms a virtual image 30.1 cm away. What is the magnification?

Have you accounted for the fact that it's a virtual image?

 

[PeterO]

Homework Statement

A converging lens with a focal length of 14.2 cm is used to inspect a coin. The lens forms a virtual image 30.1 cm away. What is the magnification?

Homework Equations

(1/Si) + (1/So) = (1/f)
M = (hi/ho) = (-Si/So)

The Attempt at a Solution

1/So = 1/f - 1/Si

1/(1/So) = So = 26.88cm

M = (-Si/So) = -30.1/26.88 = 1.12


I'm pretty sure that I have the solution and answer correct, but I just wanted to double check. This homework is online and the site is telling me that I have the answer wrong and I just wanted to make sure that I wasn't missing anything before I bother my teacher about it. As of right now I think it's just an error in the code of the site, but I'm just double checking. Thanks.

This object [the coin] is placed inside the focus [giving a virtual image] so some/all of those distances may be negative.
I am never confident with that (1/Si) + (1/So) = (1/f) formula, so I always use a couple of alternates, and a ray diagram.

 

[TSny]

Check your notes or your textbook. There are specific rules for deciding on the signs of the various quantities.

 

[fordy314]

Check your notes or your textbook. There are specific rules for deciding on the signs of the various quantities.

Ah, right, I forgot about the signs. Looks like the summer off has made me a little rusty. I just accounted for the signs and got the right answer. Thanks for your help.