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Old Jan 1st 2018, 01:00 PM   #1
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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 from close distance any reflection beyond the focal length appears upright, albeit a little blurry.

For example an object that is located at the focal point, 2600mm away from the mirror, appears upright, not inverted. Ray diagrams shows is that a real image is formed and it is located at 2600mm from the mirror as shown in the photo



The real image is inverted. Since I am looking at the mirror from very close there is no way from me to see the real image. The reflection I am seeing at the mirror must be a virtual image behind the mirror. But I can't see how a virtual image is formed using simple ray diagrams. Does anybody have an idea of what is a happening?
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Old Jan 2nd 2018, 02:57 PM   #2
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Draw lines from the object to the eye via the mirror,
Now where the lines meet the mirror, continue the lines back behind the mirror,
This models the optical illusion that the image is behind the mirror.
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Last edited by Woody; Jan 2nd 2018 at 03:00 PM.
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Old Jan 3rd 2018, 01:03 AM   #3
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Originally Posted by nikosb View Post
If an on object is located beyond the focal length of a concave mirror then the virtual image should appear inverted.
No ....if the eye is closer to the mirror than the focal point the image won't be inverted ..

In simple terms ,it's when light from a distant object hits the mirror and then passes through the FP that it is 'flipped over' and gives an inverted image .

Your diagram is in error .The eye will not see the object shown , it can not see the the top blue line , or it's reflection in the mirror.

To understand this look at the ray diagram for a plane mirror , since your FP is so long it's almost the same . Then imagine how this would change if the mirror had a very slight concavity .

Last edited by oz93666; Jan 3rd 2018 at 01:22 AM.
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