Why virtual images in telescopes & microscopes?

In summary, the ocular distance in telescopes and microscopes is designed to create virtual images instead of real images by presenting an image that appears to be at infinity. This allows the eye to be relaxed while viewing and reduces eye strain. Placing an object at the focal point of a positive lens will result in parallel rays that can be easily viewed by the eye. However, some sources may state that no image is formed and only a blur is seen due to the undefined case of infinite magnification. This is because the magnification is angular and the angle subtended by the rays from the image is greater than the angle subtended by the object on its own.
  • #1
Marketo
10
0
Why in telescopes and microscopes the ocular distance is such that it creates virtual images instead of real images?
 
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  • #2
The rays that come through the eyepiece lens, and appear to be coming from a virtual image are diverging. A normal eye can accommodate (focus) diverging rays, so can 'see' the virtual image, even if it is only a few cm from the eye.

Each point on a a real image is formed by converging rays. A normal eye intercepting such rays can't accommodate them (you hardly ever get converging rays in the natural world). You can, of course, see a real image on a screen, because light is scattered from the screen, giving rays diverging from each point. But forming and viewing such a real image would have no advantages and plenty of disadvantages.
 
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  • #3
Ok, thanks, it also would be interesting to know why the eye only just can accommodate diverging rays.
 
  • #4
Well, as I said, you hardly ever get converging rays in the natural world, so there is no need for the eye to be able to do this. [In Darwinian terms, no natural selection for the evolution of an eye which can do this.]
 
  • #5
Marketo said:
Why in telescopes and microscopes the ocular distance is such that it creates virtual images instead of real images?

The function of the eyepiece is to present an image to your eye that appears to be at infinity. This is so your eye is relaxed, making viewing easier and reducing eye strain. Because the image appears to be behind the eyepiece, the image plane of the eyepiece is on the 'wrong' side of the lens and is thus a virtual image.

Eyepieces are complex optical devices- very asymmetric and difficult to design. In addition to a virtual image, the exit pupil of the eyepiece is outside of the lens- it is located at the pupil of your eye when viewing.
 
  • #6
hello forum,
this leads me to a question:

if an object is placed exactly on the focal point of a positive lens, will an image form?
I think so, because the converging lens inside our eyes will be able to focus those parallel rays and form a real image on the retina.

But some people told me that we would only see a blur instead: the focal point is where a real and virtual image should form at the same time so a blur is what we end up seeing.
I was told to perform an experiment with a concave mirror: as we move the object towards the mirror the image is real until we get exactly on the focal point where we see a blur.
If we move the object past the focal point the image become erected and virtual...

Thanks,
fisico30
 
  • #7
At its most relaxed the (normal) eye is set to focus on infinity, that is to accept parallel light rays.
An object placed at the focal point of a converging lens will produce parallel rays and will therefore be easily viewed by the eye.
This is a magnifying glass in normal use.
I would agree with Philip woods descriptions... clear and to the point
 
  • #8
Thank you technician!

I am always bugged when the certain notes say that the lateral magnification is infinite, the image location is infinite and no image is formed (undefined case) and blur is seen when the object is on the focal plane...

But I agree with you and the others: the lens in the eye focuses that bundle of parallel rays and an image is actually seen!

fisico30
 
  • #9
The magnification is ANGULAR. The angle subtended by the rays from the image is greater than the angle subtended by the object on its own.
 

Related to Why virtual images in telescopes & microscopes?

1. Why do telescopes and microscopes use virtual images?

Telescopes and microscopes use virtual images because they allow for magnification and manipulation of the image without physically moving the object being viewed. This is especially important for astronomical and microscopic observations where the object is too far away or too small to be seen with the naked eye.

2. How are virtual images created in telescopes and microscopes?

Virtual images in telescopes and microscopes are created through the use of lenses or mirrors. These optical components bend and focus light rays to create an enlarged and magnified image of the object being viewed. The image appears to be in a different location than the actual object, hence the term "virtual".

3. Can virtual images be seen without the use of a telescope or microscope?

No, virtual images can only be seen through the use of optical instruments such as telescopes and microscopes. This is because the lenses and mirrors in these instruments are specifically designed and positioned to create a virtual image at the eyepiece or viewing point.

4. Are virtual images in telescopes and microscopes always upright?

Not necessarily. The orientation of the virtual image depends on the type of lens or mirror used in the instrument. For example, a convex lens will create an upright virtual image, while a concave lens will create an inverted virtual image. Mirrors can also produce different orientations of virtual images depending on their curvature.

5. What are the advantages of using virtual images in telescopes and microscopes?

There are several advantages to using virtual images in telescopes and microscopes. First, they allow for magnification of small or distant objects, making them easier to study. They also allow for manipulation of the image through the use of different lenses or mirrors. Additionally, virtual images are not affected by the physical properties of the object being viewed, such as color or texture, allowing for a clearer and more focused image.

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