# I About a concave mirror with a large focal length (1000mm)

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1. Dec 31, 2017

### nikosb

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 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 appear 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?

2. Dec 31, 2017

### davenn

do a ray diagram of your setup, do some googling if you need to understand how to do that

3. Dec 31, 2017

### nikosb

I have google it and a ray diagram for an object located beyond the focal length of a concave mirror should produce an inverted image like shown below.

What happens in my case is that the image is upright. So I am guessing that the focal length listed for this mirror refers to something else?

4. Dec 31, 2017

### sophiecentaur

You can find the focal length of any mirror (within reason) by focussing a distant light on a white card and measuring the distance. You can easily check the advertised figure. The "image flip" to upright will be for short object distances. There are hundreds of images of this on Google.

5. Jan 3, 2018

### nasu

The image produced by the. Mirror alone is real and not virtual. To see it like this you need to catch it on a screen. What you see without screen is the result of the mirror and the lens in your eye, I suppose.

6. Jan 3, 2018

### jbriggs444

If you position your head between the mirror and the plane where the real image will form then the effect would indeed be to see an upright image positioned "beyond infinity". It would be difficult to focus on because your eyes are not built to focus anywhere past infinity.

This should not be surprising. It's just like looking in an ordinary flat mirror that's not quite flat. You look up and you see stuff that's higher up. You look down and you see stuff that's farther down.

If you were to move your head back further toward the image plane, there would be a magnifying effect. Less and less of the object would fill the available viewing angle. As your eyes moved into the image plane only a tiny area on the object could remain visible. This portion would fill the entire mirror area with a blur. [Actually two points on the object and two blurs -- one for each eye]

As your eyes moved further back from the image plane, you would be in a position to see the inverted real image. Until your eyes were a few inches back, this image would likely be out of focus and unavailable for binocular vision unless you are both myopic and cross-eyed.

[Note that it can be fun de-coupling binocular vision from focus. I spent days and weeks in my teen-age years practicing on pegboard and at age 60+ can still go cross-eyed at will. It makes solving those "what's different in these two scene's" puzzles a total breeze. Much harder is getting one eye to track down while the other tracks up. I never did get the hang of that and can only do a couple of degrees]

7. Jan 3, 2018

### sophiecentaur

I thought that sort of thing was the result of certain teenage male practices.
But I do know what you are talking about. It helps me to 'solve' autostereograms.

The projection method is the sensible way to locate the focus of a mirror or a lens. Distant street lamps are suitable (do not project the Sun without a lot of care if the reflector is more than a very few cm diameter.