Physics Review Question

Unit: Optics
Year: 1994 Question#: 89
Question: A concave mirror with a focal length of 20.0 centimeters is used to examine a 0.50-centimeter-wide freckle on a person's face. The person's face is located 10.0 contimeters from the mirror.

The image of the freckle produced by the mirror is...

(1 ) real and inverted
(2 ) real and erect
(3 ) virtual and inverted
(4 ) virtual and erect

What is this question really asking?

Explanation by: Jordan L.

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Answer 1
1) real and inverted

This answer is entirely INCORRECT.

Answer #1 is incorrect. This is made clear by looking at this ray diagram of the question. If the image produced is behind the mirror, as in this case, then it means that the image is virtual, not real. The image is oriented the same way as the object (the person's face)itself, so there's no way it's inverted.

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Answer 2
2) real and erect

This is freaking INCORRECT.

 This answer is half right, but it's just not all there. According to the diagram, the image is virtual (because it appears behind the mirror) and it is erect. While the image of the person's face is erect, it is not real, so this answer is wrong.
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Answer 3
3) virtual and inverted

Another INCORRECT answer.

 O.K. So we've established that the image of the face is virtual, going by the diagram, so I'm giving props for Answer three to have this part right. You can tell it's virtual because the image appears behind the mirror. You know, I think it's time to give up this
"Inverted" garbage. Clearly, this excellent ray diagram shows that the image is oriented the same way as the person's face is. It's NOT an inverted image.

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Answer 4
4) virtual and erect

Yeah ! Finally the CORRECT answer.

Once again I'm pulling out the same ray diagram to explain why this answer is right. The image is virtual, because it appears behind the mirror, and it is also erect, because it is oriented the same way as the person's face. If you picked Answer #4, then you got it right and can move on.

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What's this question really asking?

This question is asking you to use a diagram to know the priciples of a concave mirror. return to top

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