Image length of infinity?

1. Jan 25, 2014

Optiks

Let arrow height be 1cm.
focal length:1cm
So if I move the arrow to the focal point,image is not formed.(i.e formed at infinity)
But,
Lets move it to say 0.999
Now the image is virtual and is magnified to about 1000X
If I make it closer and closer to the focal point,the image will eventually be infinite in length
How is this possible?
Then what's the need for microscopes.

2. Jan 25, 2014

Staff: Mentor

An image that is at an infinite distance is not really useful, as you cannot watch it.
An image at a very large, but finite distance is possible. That's exactly the way a simple microscope works.

A better microscope will use more lenses to correct for various technical difficulties (no lens is perfect, they react a bit different depending on the angle and color of incoming light and so on), but the basic idea is always the same.

3. Jan 26, 2014

Ok. Now keep the settings as previously stated(but keep the object at the focal point) and imagine that you are putting your eye in at the focal length of the right side of the lens.
Will you see anything at the edge of the lens?
Even if the rays are coming parallel to your eye,you would still see some color.
But no rays are coming towards your eyes in that direction.

4. Jan 26, 2014

mfb,You didn't really explain this point.

5. Jan 26, 2014

Drakkith

Staff Emeritus
What do you mean "at the edge of the lens"?

With the arrow 1 cm away from a lens with a 1 cm focal length the light rays leaving the lens on the right side will be parallel to themselves. If you put your eye in the light path you will see an image of the arrow. However, the further away your eye is from the lens, the larger the distance between points on the image gets, and eventually not all of the light rays can enter your eye and you will not be able to see the entire arrow anymore.

As you move the object closer to the focal point, the rays converge/diverge less and less until they are finally parallel, which is when the image will be infinite in length. Parallel light rays do not form an image of any type. Does that help? If not, what exactly is confusing about the situation?

6. Jan 26, 2014

I mean the "edge".The place furthest from the center of the lens.

If I bring the image to 0.999999999999999 cm,the image height will be x.I don't want to move it to 1cm.Just close to it.So,as you move it closer and closer and closer,the image will be larger than the solar system.
How is this possible???

EDIT:The program will crash if you write that value.But think logically.

7. Jan 26, 2014

Drakkith

Staff Emeritus
Yes, I know where the edge of the lens is. What I mean is that I don't know what you are asking when you ask, "Will you see anything at the edge of the lens".

What's the problem? That just means that the focal length of the rays is so long that by the time the light converges to a point the distance between the ends of the image is larger than the solar system. What exactly are you confused about?

8. Jan 26, 2014

Ok.Let's leave it.I first want the one below.

I didn't mention anything about increasing the focal length.Focal length is 1cm and the object is brought closer to it.So image length is increased tremendously.Why can't we build a microscope with this?We will bring the object really closer to the focal length by the means of a computer or something.
This is my confusion.
Look at the simulation link I provided.

9. Jan 26, 2014

Staff: Mentor

Why do you expect anything special at the edge?
You'll see light, but you will be unable to recognize objects as their image is totally distorted.
Which direction, and why?

I don't see the problem. You'll need a screen the size of the solar system, and it won't work with visible light due to diffraction, but that is a different topic.

10. Jan 26, 2014

Drakkith

Staff Emeritus
The focal length of the lens is unchanged. The focal length of the light rays, by that I mean the distance the light travels before it is brought to focus, is enormous.

11. Jan 27, 2014

I think you all have misunderstood.
I am talking about virtual rays. 0.999999 cm is closer than focal length isn't it?

Last edited: Jan 27, 2014
12. Jan 27, 2014

Drakkith

Staff Emeritus
In the case of virtual rays it would be the "virtual" focal length of the rays. The diverging cone of light looks like it is coming from a point X cm's in front of the lens, where X is the image displacement in your program. At 0.9999999 cm, the image displacement is very, very far away.

13. Jan 27, 2014

And Image gets large and will look very large?(Like magnified 10000X)?

14. Jan 27, 2014

Staff: Mentor

The image is indeed very large, but how far away is it?

When you look at an object or image, its apparent size depends both on its actual size and on its distance from you. Try calculating the angular size θ of the image, where

tan θ = (height of image) / (distance of image from your eye)

Try it for different object locations, e.g. 0.9 cm, 0.99 cm, 0.999 cm.

Last edited: Jan 27, 2014
15. Jan 27, 2014

Drakkith

Staff Emeritus
Yes, but beyond a certain magnification the image just becomes a blurry mess. I just tried it using a telescope eyepiece, and the virtual image had a limit to how big it could get before I could no longer see any detail.

16. Jan 28, 2014

I pu my eye in the focal point
Yes in that way,if the focal length is unchanged,the angular size is always the same.If your eye is in the same place.So the image should always look the same size.But this is not the case.Am I getting something wrong?

Last edited: Jan 28, 2014
17. Jan 28, 2014

Varun Bhardwaj

It depend on the quality and size of lens that how much you can magnify image on the edge of lens
If you want to magnify a image infinite than you require infinite quality and size of lens,that is not possible
It is thing to think but not to do really.

18. Jan 28, 2014

Am I dong it?Or going to do it?
I am just thinking.Ya know..

19. Jan 29, 2014

20. Jan 30, 2014

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