Image formed from object at focal point of concave mirror

  • #1
The question posed: "An object is located to the right of a mirror that in concave in its [the object's] direction. If the object sits on the focal point of the mirror, what answer best describes the image formed?"

Our class is united in the notion that as the rays would be parallel, intersecting only at infinity, no image is formed on either side of the mirror (or at infinity on both sides). Our textbook corroborates this, but our professor has only offered four answer choices:

A. The image is to the left of the mirror and larger than the object.
B. The image is to the left of the mirror and smaller than the object.
C. The image is to the right of the mirror and larger than the object.
D. The image is to the right of the mirror and smaller than the object.

and maintains that the answer is B, which would indicate--to assume I've read the setup correctly--a virtual image, being to the left of and therefore behind the mirror, with a negative magnification. Does the solution lie in the wording of the question ("best describes") or are we all missing a key factor in our understanding of spherical reflecting surfaces?

I'd call it a mistake on her part, but she seems entirely sure of herself and undergraduates are nigh-guaranteed to be outclassed in optics knowledge by a full-time optoelectronics researcher with a Ph.D in experimental solid-state physics. Really appreciate the help, folks!
 

Answers and Replies

  • #2
Simon Bridge
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Ask for a demonstration of the working.
The model answer is correct for the mirror convex in the objects direction... may be a typo on the question.
 

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