放大鏡和望遠鏡一問

[dwond]

這個情況下, 看不到影像, 應該要指明是投影到物體另一側的實像, 而不是放大鏡後(與物體同側)的虛像.
放大鏡後的虛像是肉眼可以看得到的.

實像: 實際上可以投在屏幕上的影像
虛像: 不可以投在屏幕上的影像

中學物理課老師教的......

路過...... [樂奔]

無論虛像實像, 成眼內都可能有成像.
因為人眼有多"一塊lens".
問題只是眼睛的lens 焦距合不合適而已.

不可以投在屏幕上的影像,
指的是直接投影在屏幕 (即沒有另一塊lens).

[Chanlunlun]

想請教大家一條可能是好簡單的問題，但我想不到答案，間中就有學生問。

問題是，當一個convex lens被用作放大鏡，而我把物體放在焦距的距離 (i.e. lens formula: set u=f)，像當然會在infinity出現。現實中，用放大鏡作測試，為什麼我們卻看不到一個清晰的影像？

再者，其實telescope的ray diagram中，eyepiece那半的ray diagram就和放大鏡u=f的情境一樣，為什麼telescope便看到清晰的像，convex lens便看不到?

麻煩大家幫忙。

Dear David,

Let me try to solve this problem, once and for all.

The diagram of the textbook is WRONG. The ray diagram is all right but the attached picture on right hand side is completely incorrect.

If the emergent rays are parallel we MUST see a sharp, magnified and erect image, just like what we see inside the eyepiece of a telescope.

Then, why, in the book, and in some of our experiment, we always see a blur when the image transits from virtual to real ?

The answer is that when we THINK we SAW this transit we WERE always too late to see it occur. Put it in simple words - when we saw the whole lens blurred, the lens was already forming a real inverted image but it just happened that the image was formed at the lens of our eyes so we cannot focus. Consequently we saw the who lens blurred out.( remember Foucakt test? ) This could only happen when u > f and our eyes ( or the camera lens ) situated exactly at the image plane.

Then what will we see at the real transit ? When the image forms at the infinity, we will see a sharp, upright and magnified image. When u is just a liitle bit bigger than f, transit occurs and we still see an upright and magnified image, but not very sharp since our eyes cannot focus convergent light beam. At this moment, with the lens and object at fixed position, try to move your eye further and further away from the lens, you will see the image turns from upright into inverted because your eye moves from inside the image plane to outside the image plane. When your eye is exactly at the image plane, you see the whole lens blurred out just as what the picture shows.

When image is formed exactly at infinity, the image ALWAYS REAMINS SHARP no matter where you situate your eye.

Textbooks, especially those used in secondary school, are not always correct.

Best regards
Chan Yuk Lun

[David]

Thanks again for all helpful comments.

[物理小子]

想請教大家一條可能是好簡單的問題，但我想不到答案，間中就有學生問。

問題是，當一個convex lens被用作放大鏡，而我把物體放在焦距的距離 (i.e. lens formula: set u=f)，像當然會在infinity出現。現實中，用放大鏡作測試，為什麼我們卻看不到一個清晰的影像？

再者，其實telescope的ray diagram中，eyepiece那半的ray diagram就和放大鏡u=f的情境一樣，為什麼telescope便看到清晰的像，convex lens便看不到?

麻煩大家幫忙。

Dear David,

Let me try to solve this problem, once for all.

The diagram of the textbook is WRONG. The ray diagram is all right but the attached picture on right hand side is completely incorrect.

If the emergent rays are parallel we MUST see a sharp, magnified and erect image, just like what we see inside the eyepiece of a telescope.

Then, why, in the book, and in some of our experiment, we always see a blur when the image transits from virtual to real ?

The answer is that when we THINK we SAW this transit occurs we WERE always too late to see it occur. Put it in simple words - when we saw the whole lens blurred, the lens was already forming a real inverted image but it just happened that the image was formed at the lens of our eyes so we cannot focus. Consequently we saw the who lens blurred out.( remember Foucakt test? ) This could only happen when u > f and our eyes ( or the camera lens ) situated exactly at the image plane.

Then what will we see at the real transit ? When the image forms at the infinity, we will see a sharp, upright and magnified image. When u is just a liitle bit bigger than f, transit occurs and we still see an upright and magnified image, but not very sharp since our eyes cannot focus convergent light beam. At this moment, with the lens and object at fixed position, try to move your eye further and further away from the lens, you will see the image turns from upright into inverted because your eye moves from inside the image plane to outside the image plane. When your eye is exactly at the image plane, you see the whole lens blurred out just as what the picture shows.

When image is formed exactly at infinity, the image ALWAYS REAMINS SHARP no matter where you situate your eye.

Textbooks, especially those used in secondary school, are not always correct.

Best regards
Chan Yuk Lun

but how about marking schemes in HKEA?
which would it use? blur or clear for object distance = f?
any past paper as example?

[Subaru]

Dear David,

Let me try to solve this problem, once for all.

The diagram of the textbook is WRONG. The ray diagram is all right but the attached picture on right hand side is completely incorrect.

If the emergent rays are parallel we MUST see a sharp, magnified and erect image, just like what we see inside the eyepiece of a telescope.

Then, why, in the book, and in some of our experiment, we always see a blur when the image transits from virtual to real ?

The answer is that when we THINK we SAW this transit occurs we WERE always too late to see it occur. Put it in simple words - when we saw the whole lens blurred, the lens was already forming a real inverted image but it just happened that the image was formed at the lens of our eyes so we cannot focus. Consequently we saw the who lens blurred out.( remember Foucakt test? ) This could only happen when u > f and our eyes ( or the camera lens ) situated exactly at the image plane.

Then what will we see at the real transit ? When the image forms at the infinity, we will see a sharp, upright and magnified image. When u is just a liitle bit bigger than f, transit occurs and we still see an upright and magnified image, but not very sharp since our eyes cannot focus convergent light beam. At this moment, with the lens and object at fixed position, try to move your eye further and further away from the lens, you will see the image turns from upright into inverted because your eye moves from inside the image plane to outside the image plane. When your eye is exactly at the image plane, you see the whole lens blurred out just as what the picture shows.

When image is formed exactly at infinity, the image ALWAYS REAMINS SHARP no matter where you situate your eye.

Textbooks, especially those used in secondary school, are not always correct.

Best regards
Chan Yuk Lun

鏡神即係鏡神, 我都發覺我生銹的concept錯了.

因為我漏咗睇個pseudo object原來在無限遠, 所以無限大的角放大率並不成立.

其實在放大鏡的case中, angular magnification是因眼睛在不同的距離(assume沒有放大鏡在之間)而有所改變(Theta'不變, 但Theta變). 假如用放大鏡把物件放在f, 視大小就等於把眼睛放在f的距離看(假如能focus到).

我想吓, 其實放大鏡的"真正"目的並不是用來放大, 而係幫助眼睛focus到比minimium focusing distance還短的距離, 功用跟add on marco鏡一樣.

[Chanlunlun]

[but how about marking schemes in HKEA?
which would it use? blur or clear for object distance = f?
any past paper as example?

Dear Mr/Miss Physics Kid,

Don't worry !!

The marking scheme must be according to what I have said - the image must be upright and clearly seen when u = f , no matter where are your eyes. Buy a magnifying glass and do yourself an experimrnt.

Textbook could always be wrong, but seldom does the marking scheme.

Best regards
Chan Yuk Lun