# Thread: I'm really lost - Concave mirrors and reproduction of an actual object

1. ## I'm really lost - Concave mirrors and reproduction of an actual object

hiya

I'm really having some problems understanding concave mirrors and their reproduction of actual object and I'm hopping you can give me some thorough explanation about the subject at hand

Suppose that a light bulb is placed in front of a concave mirror at a location somewhere behind the center of curvature (C). The light bulb will emit light in a variety of directions, some of which will strike the mirror. Each individual ray of light which strikes the mirror will reflect according to the law of reflection. Upon reflecting, the light will converge at a point. At the point where the light from the object converges, a replica or reproduction of the actual object is created; this replica is known as the image. Once the reflected light rays reach the image location, they begin to diverge . The point where all the reflected light rays converge is known as the image point. Not only is it the point where light rays converge, it is also the point where reflected light rays appear to an observer to be diverging from. Regardless of the observer's location, the observer will see a ray of light passing through the real image location. To view the image, the observer must line her sight up with the image location in order to see the image via the reflected light ray. The diagram below depicts several rays from the object reflecting from the mirror and converging at the image location.

1)

Can you show me some proof as to why all rays of light from the object converge at the same point ( image point )?

2)

I'm having hard time understanding how person looking at real image of an object ( created by concave mirror ) will perceive this image?
Will person have the feeling as if real image ( aka reproduction of an actual object ) is located in front of a concave mirror?

*To explain myself further: plane mirrors create virtual images --> images which are formed in locations where light does not actually reach, even though it appears to an observer as though the light were coming from position somewhere behind the mirror. So an image of an object is actually
" created on the surface of a mirror " --> mirror in a way acts as monitor screen.

But with concave mirrors, it sounds to me as if image of an object is created away from the mirror, thus unlike with plane mirrors ( which have image of an object created on its surface, even if it appears to observer as if image is located behind the mirror ), concave mirrors don't have an image of object created on its surface?

3)

BTW-How does one perceive something as being closer or further away? Does it have to do with size of an object ( meaning if object appears bigger than it is closer ) or is there more to it?

thank you

2. Originally Posted by boris16
...
1)Can you show me some proof as to why all rays of light from the object converge at the same point ( image point )?
2)I'm having hard time understanding how person looking at real image of an object ( created by concave mirror ) will perceive this image?
Will person have the feeling as if real image ( aka reproduction of an actual object ) is located in front of a concave mirror?
on (1) No they do not all converge at one point. This is called "sperical aberation" The relative locations for source and image you show are better focused by an "off axis parabolid mirror."
Originally Posted by boris16
...But with concave mirrors, it sounds to me as if image of an object is created away from the mirror, thus unlike with plane mirrors ( which have image of an object created on its surface, even if it appears to observer as if image is located behind the mirror ), concave mirrors don't have an image of object created on its surface?
The rays from the source are diverging. If the concave curvature is not sufficient to make them convergent after reflection the image will still b e up-right and virtual, but appear to be farther behind the mirror and smaller than if the mirror were flat. This also answers your (3).
Originally Posted by boris16
...thank you
You are welcome. Wish they were all this easy. Nice illustration, BTW.

3. Can you show me some proof as to why all rays of light from the object converge at the same point ( image point )?
It is due to the nature of the concave mirror. Hint: similar to the reason why all points on a circumference (mirror) are exactly the same distance to the center (image).

I'm having hard time understanding how person looking at real image of an object ( created by concave mirror ) will perceive this image?
Will person have the feeling as if real image ( aka reproduction of an actual object ) is located in front of a concave mirror?

Yes, it will look like a real image. I saw this experiment once in college.

BTW-How does one perceive something as being closer or further away? Does it have to do with size of an object ( meaning if object appears bigger than it is closer ) or is there more to it?
It is partly due to our previous understanding of the expected dimensions of the object and its retinal size. And partly due to our binoccular vision and the divergence of light rays from the surface of an object.

4. boris16:

Originally Posted by boris16
Can you show me some proof as to why all rays of light from the object converge at the same point ( image point )?
It follows from the laws of reflection - angle of incidence = angle of reflection - and the fact that the mirror is spherical.

I'm having hard time understanding how person looking at real image of an object ( created by concave mirror ) will perceive this image?
They perceive it exactly as they would if the object was actually located at the image location.

*To explain myself further: plane mirrors create virtual images --> images which are formed in locations where light does not actually reach, even though it appears to an observer as though the light were coming from position somewhere behind the mirror. So an image of an object is actually
" created on the surface of a mirror " --> mirror in a way acts as monitor screen.
That last sentence is incorrect. The image is actually created behind the mirror, but it is a "virtual" image rather than a real image. There is no "image" on the mirror.

With a concave mirror, the image is created in front of the mirror instead of behind it.

BTW-How does one perceive something as being closer or further away? Does it have to do with size of an object ( meaning if object appears bigger than it is closer ) or is there more to it?
There are both binocular and monocular clues. Apparent size is a monocular clue. Parallax is a binocular one, which depends on having two eyes instead of just one.

5. There is also accomodation (spelling?), the muscle sence of the degree of flexing of the lens of the eye, a monocular clue. It is very subtle and frequently is overidden by other clues, but it is there.

6. James:

Specifically: Sperical aberation is very real, and as I stated a concave mirror can make a small upright virtual image behind the mirror. (When the source is closer to the axial point of the mirror than R/2)

7. Originally Posted by kevinalm
There is also accomodation (spelling?), the muscle sence of the degree of flexing of the lens of the eye, a monocular clue. It is very subtle and frequently is overidden by other clues, but it is there.
and several more - most important are relative motions (either of the eye ball* or among the objects in view) and "texture gradients" in the field of view. (For example, even in a still photo where none of the others 3D mechanisms others have mentioned is possible, it is immediately obvious which pigeons in a flying flock are the more distant ones. - This an example of what psychologist of vision call "texture gradients.")
-------------------------------------------
*Pigeons have no overlap in the field of visions of their two eyes. - I think their constantly "bobbing" of their heads while walking is at least partially for seeing in 3D.

8. hiya

Originally Posted by John Connellan
Originally Posted by boris16
I'm having hard time understanding how person looking at real image of an object ( created by concave mirror ) will perceive this image?
Will person have the feeling as if real image ( aka reproduction of an actual object ) is located in front of a concave mirror?
Yes, it will look like a real image. I saw this experiment once in college.
But then doesn't a spoon produce a real image (since part of the spoon has the shape of concave mirror )? Meaning if I place an object infron of the spoon, then why doesn't real image of an object appear, say one meter infron of the spoon ( thus in order to see this image I wouldn't even have to look in the direction of the spoon ).

But in reallity all I see is my reflection on the surface of the spoon ( aka virtual image ).

thank you all very much for your kinda help

9. Originally Posted by Billy T

Specifically: Sperical aberation is very real, and as I stated a concave mirror can make a small upright virtual image behind the mirror. (When the source is closer to the axial point of the mirror than R/2)
Yes, you're right. Rays parallel to the optical axis of a spherical mirror don't all converge at the focus - that's spherical abberation. A parabolic mirror does the job properly.

10. Although for the conditions shown by the diagram (where both image and object are near C), a spherical mirror would have a better image than a parabolic.

11. Originally Posted by boris16
hiya

But then doesn't a spoon produce a real image (since part of the spoon has the shape of concave mirror )? Meaning if I place an object infron of the spoon, then why doesn't real image of an object appear, say one meter infron of the spoon ( thus in order to see this image I wouldn't even have to look in the direction of the spoon ).

But in reallity all I see is my reflection on the surface of the spoon ( aka virtual image ).

thank you all very much for your kinda help
Take a look at this experiment. It was the one I did in college.

http://hendrix.uoregon.edu/~demo/Dem...ical/Real.html

There could be many reasons for the spoon experiment failing but here are the main ones:

1. Most spoon are NOT spherically concave (they're all oval shaped aren't they?!)
2. Most spoons are not perfect mirrors. They do not reflect light well enough to produce clear real images.
3. The real image of a spoon is hardly going to be located 1 meter away as u say. The focal point of a spoon is a few millimeters from the inside surface of the spoon.

12. Originally Posted by James R
Yes, you're right. Rays parallel to the optical axis of a spherical mirror don't all converge at the focus - that's spherical abberation. A parabolic mirror does the job properly.
Spherical aberation is not just for "parallel light" All images have it with spherical mirror. Also you did not acknowledge your other error. Let me state the truth more strongly: For some relativel large radius of curvature mirror, the image can be on either side or the mirror at any distance from the mirror except (I think -just making rays and images in my head) between R/2 and zero in front of mirror. Specificially if the source is closer to the mirror than R/2 the image is virtual, upright and behind. Source greater than R from concave mirror has image between R and R/2. Please correct me if I am wrong.

13. If image and object are both very near C, then a sperical mirror _doesn't_ suffer from spherical abberation, but a parabolic will. If image and object occupy infinity and F (in either order), then a spherical mirror _does_ suffer from spherical abberation and a parabolic won't. Intermediate to these two extremes both suffer from SA, and precision optics are sometimes optimized for the specific conditions with odd curves.

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