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Thread: Basic question about primary mirror and focal length

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    Default Basic question about primary mirror and focal length



    I have a Cassini PM-160 spherical mirror used in telescopes as the primary mirror. The mirror is concave with a radius of curvature of 2600mm and focal length of 1300mm. I am using the mirror in an imaging application. I have a very basic question.

    If an on object is located beyond the focal length of a concave mirror then the virtual image should appear inverted. When I look at the mirror any reflection beyond the focal length appears upright, albeit a little blurry. For example an object that is located at 2meters away from the mirror appears upright, not inverted. Why is that? If the radius of curvature or focal length not advertised correctly?

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    Default Re: Basic question about primary mirror and focal length

    Welcome to the forum.


    It is the real image at the focal plane that appears inverted. You can verify this by placing apiece of paper at the focal point to catch the real image. Because your eyes are also well outside the focal distance, you are not seeing the real image, you are seeing a virtual image formed "behind" the mirror, just as you would with a flat bathroom mirror. The virtual image is right side up.
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    Default Re: Basic question about primary mirror and focal length

    Thank you very much for the reply. I experimented with the mirror and I think you are right. But all the physics websites that I've read only talk about a real image that forms when an object is located past the focal point of the mirror. None of them talks about a virtual image except when the object is located within the focal length. I am not sure how the ray diagrams would form a virtual image if the object is located past the focal point. Do you recommend a website or a textbook where I can read more about this case?

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    Default Re: Basic question about primary mirror and focal length

    So the ray diagrams for an object located at the radius of curvature form a real image that is inverted. Since the eye is located too close to the mirror I can't see the real image. But how do the ray diagrams form a virtual image?

    object focal point concave mirror.gif

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    Default Re: Basic question about primary mirror and focal length

    The concave mirror produces a virtual image in the same way that a flat mirror does. The ray diagram is essentially the same as for a flat mirror. The curvature does change the angles of the light rays, resulting in a magnified virtual image.
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    Default Re: Basic question about primary mirror and focal length

    When I trace out a ray diagram for an object that is located beyond the focal length of the mirror I can only get the formation of the real image. But if the observer is too close to the mirror the observer doesn't see the real, he/she sees a magnified virtual image. How do I trace the ray diagrams for the virtual image?

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    Default Re: Basic question about primary mirror and focal length

    This site is no longer in general use; most of the active membership is now at this site https://theskysearchers.com/

    In answer to your question, a site that I have found useful is The Physics Classroom, in this instance here https://www.physicsclassroom.com/cla...oncave-Mirrors
    Mary


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