# Which path does light take through a lens?

In the picture below, the three principal light rays are drawn refracting through the lens , forming an image on the other side. But which path does the light actually take? That is , if i place my eye where the image is said to be forming will I see light coming from the top ray, the middle ray or the bottom ray? It seems to me that if the light is coming from the top ray, it would appear that the image is coming from a higher altitude than if the light was coming from the bottom ray.

I have drawn in where i would expect to see the image coming from in this image here and this leads me to believe that if I placed my eye where the image is said to be formed i should somehow be seeing double?

Any help on this would be most appreciated!

I am afraid that you have misinterpreted the ray diagrams which are there to show you the position and nature of the real image formed by the lens.

In reality there are many more rays which produce the image as shown in the diagram below and described in my answer to the question, Why do you need at least two rays to form an image?.

. . . . . if i place my eye where the image is said to be forming . . . . .
you will not see an image rather you will experience the result of bright? light entering you eye.

You need to consider the diverging rays carrying on past the image and entering the eye.

After refraction by the optical system of the eye a real image of the intermediate real image formed by the lens is formed on the retina of the eye.

In order that the eye produces a sharp focus of the intermediate onto the retina the distance between the eye must be greater than the least distance of distinct vision which is about $$250\,\rm mm$$ for the "standard" eye.
So putting your eye where the intermediate image is will mean that the eye will be unable to produce a sharp focus image of the intermediate image.
I hope this also dispels any idea of the eye being able to differentiate between the top, middle and bottom construction ray.

Fianlly you may care to read my answer to the post, Real images and their formation, which tries to explain the formation of a real image.

• Thank you so much, this is honestly a brilliant answer. Just to check my understanding though, If i had the set up in this image (i.imgur.com/3NxGtOp.png) If my eye were the yellow ellipse the image would be blurry but if my eye were the green ellipse the image would be clear right? – SalahTheGoat Aug 19 '20 at 8:18
• (part 2) My only confusion now lies in if my eye is placed at the red ellipse. Does the eye perceive all rays emanating from a single point as a single wave and hence the image does not appear blurry ? Because if I apply ray optics to this normal (non lens) situation then I'm led to believe that the eye will always produce blurry images since rays from the same point arrive at different locations in the eye – SalahTheGoat Aug 19 '20 at 8:19
• First question - yello/green ellipse, correct. Second question - red ellipse if you looked towards the lens then you would not see the real image formed by the lens as there are no rays travelling in the direction of your eye but if you looked towards the object you would see the object clearly if your eye was further than the least distance of distinct vision. – Farcher Aug 19 '20 at 9:07
• Yes sorry, for the second question I meant looking towards the object. Thanks for the fantastic answers though. They've helped immensely! – SalahTheGoat Aug 19 '20 at 13:36

if i place my eye where the image is said to be forming will I see light coming from the top ray, the middle ray or the bottom ray?

All of them, and all the other rays in between that aren't shown in the diagram.

And the fact that all of them end up in the same place on your retina is why you see a clear image of the object.

It seems to me that if the light is coming from the top ray, it would appear that the image is coming from a higher altitude than if the light was coming from the bottom ray.

Your eye can't tell what angle the light that strikes the retina came from. It can only tell which parts of the retina are struck by more or less light of different colors.

• Apt username for the answer :) – DanDan0101 Aug 19 '20 at 6:58
• The OP is not looking at the image; their eye is located at the image... – DJohnM Aug 19 '20 at 7:01
• Okay this actually makes a lot of sense except as is the case often in physics I think it opens up new questions for me. If I place my eye at the red circle in this diagram (i.imgur.com/9fsFrbn.png) it should be blurry for me because light rays from the same point are hitting my retina a diferent locations. Not only that but light rays from numerous points are hitting my retina at the same point? – SalahTheGoat Aug 19 '20 at 7:21
• @DJohnM if the eye was at the image, it would see the same color in all directions. The image needs to be at a distance the eye can focus at. – John Dvorak Aug 19 '20 at 7:42