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The thin lens equation shows that when there is an object further than the focal point, there is a real image formed on the other side of the lens, and this principle is used for cameras, eyes, etc.

However, when I take a converging lens(that I got from some mobile VR headset), and hold it between me and an object, both of whom are more than the focal length away from the lens, I see an upside-down virtual image of the object.

So my question is, why do I see a virtual image instead of a blur that I would see at the point where the real image forms? What would the ray diagram look like for this?

Here is a picture of this phenomenon:

Image

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    $\begingroup$ How do you know it is a virtual image? If a single converging lens crates an upsidedown image then it has to be a real image. $\endgroup$
    – BioPhysicist
    Sep 30, 2020 at 2:08
  • $\begingroup$ It's a virtual image because I can see an upside-down smile appearing as if it is coming from the other side of the lens, just like how I can see a virtual, magnified image of the source if I decrease the distance of the lens to the screen to be less than the focal distance of the lens. $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 2:18
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    $\begingroup$ If you're getting an inverted virtual image then it's not a single converging lens. So something isn't right here. $\endgroup$
    – BioPhysicist
    Sep 30, 2020 at 2:23
  • $\begingroup$ I have tried the same with another biconvex lens from cheap google cardboard with the same effect. A magnifying glass has the same effect. I just finished studying for SAT Physics, but I was confused as to how this effect is explained by the ray model, which is why I asked this question... $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 2:30
  • $\begingroup$ @BioPhysicist I believe OP is confused about the definition of a real image. Based on comments in my answer, OP doesn't believe the definition. $\endgroup$
    – Bill N
    Sep 30, 2020 at 3:06

2 Answers 2

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A converging lens (convex lens) will always create a real inverted image if the object is located at a point beyond the focal point (at a place on the opposite side of the lens). The only exception to this, is if the object is closer (to the lens) than the focal point is to the lens, in which case the image will appear to be on the same side of the lens as the object and the image will be a right side up and it will be a virtual image.

Just because you are not seeing the image at the exact point where the rays converge, does not mean it is a virtual image. This just means that image is out of focus.

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  • $\begingroup$ What I still don't understand is why does it look like a virtual image? $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 3:06
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    $\begingroup$ @Sagar Patil What do you think a virtual image looks like? $\endgroup$
    – joseph h
    Sep 30, 2020 at 3:07
  • $\begingroup$ From what I understand, a virtual image is an image formed behind the lens due to rays appearing as starting from a virtual point. $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 3:22
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    $\begingroup$ I don't know what you mean by "a virtual point". The rays start from the object and form an image on the other side of the lens - a real image. $\endgroup$
    – joseph h
    Sep 30, 2020 at 3:25
  • $\begingroup$ But how would I be able to see a real image? I thought they can only be projected onto screens? How do the rays of a real image interact with the lens of the eye to project an inverted image of the object? $\endgroup$
    – Sagar Patil
    Oct 1, 2020 at 4:39
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You are seeing the real image, not a virtual image. If you placed a screen at the location where you believe the image to be it would be projected on the screen. Your eyes are focusing for the distance from you to that image. A screen is not necessary.

This is similar to the optical illusion of a small object like a coin or statue in a toy concave mirror setup. Your eyes see a real image, so real looking that you want to touch it, yet your fingers pass through the image. Here's an example.

The image is real, by definition, because rays from points on the object actually converge at points in space where you are looking. For virtual images, the rays do not actually converge at the image location.

EDIT: If you put your eye at the location of the image, you are adding another optical element. Your eye is not capable of focusing at distances less than about 3 cm, so you must move beyond the image location in order for your eye to have focus on the image.

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  • $\begingroup$ When I placed a screen in front of the lens, a real image formed, yes. However, when I look through the lens at that same point, I cannot see any real image, just a blur of light. But when I move my head back, I see an inverted virtual image. $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 0:51
  • $\begingroup$ @SagarPatil See my edit. $\endgroup$
    – Bill N
    Sep 30, 2020 at 0:55
  • $\begingroup$ But it's still not a real image? My eye wouldn't be able to see it if it was. What does the ray diagram for this virtual image look like? $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 0:57
  • $\begingroup$ @SagarPatil It is a real image which you are viewing because the light rays actually pass through the location as I stated in my answer. I don't have the software to do drawings like that. $\endgroup$
    – Bill N
    Sep 30, 2020 at 0:59
  • $\begingroup$ But it's not? Our eyes can't see a real image unless it is projected onto a screen? The image is my question is a virtual image by definition. I was just wondering how and why that virtual image forms. $\endgroup$
    – Sagar Patil
    Sep 30, 2020 at 2:00

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