What Do You See When Replacing a Screen with Your Eyes for a Real Image?

[pivoxa15]

Normally we see a virtue image after light from the object reflects off a mirror. We then trace back to behind the mirror to a point where the virtue image came from.

A real image is seen when light from the object projecting the image reflects off a screen or mirror and we see it with our eyes. We know where the image came from and it sits on the screen or mirror.

With the real image scenario, what happens if I try to see the light projecting the image directly? In other words replace the screen or mirror with my own eyes. What do I see? One thing is that a second pair of eyes looking at my eyes will see the image reflected off my eyes although faintly.

 

[Danger]

Can you clarify the question a bit? Are you talking about looking into a projector beam? If that's the case, you'll pretty much just be temporarily blinded.

 

[russ_watters]

Objects reflect light in all directions at once (typically). So you'll see the exact same thing whether you are 10 feet in front of a mirror or 10 feet behind where it was (minus seeing yourself, of course). But the path the photons take to get to your eye will be different.

It is simple enough to visualize if you draw a diagram of where the rays go.

 

[EmilK]

Normally we see a virtue image after light from the object reflects off a mirror. We then trace back to behind the mirror to a point where the virtue image came from.

A real image is seen when light from the object projecting the image reflects off a screen or mirror and we see it with our eyes. We know where the image came from and it sits on the screen or mirror.

With the real image scenario, what happens if I try to see the light projecting the image directly? In other words replace the screen or mirror with my own eyes. What do I see? One thing is that a second pair of eyes looking at my eyes will see the image reflected off my eyes although faintly.

Just because the concept is called 'real' doesn't mean it actually is an image of you in reality. The Physics Classroom has a thorough tutorial on [url=[PLAIN]http://www.physicsclassroom.com/Class/refln/reflntoc.html[/URL].

 

[pivoxa15]

Can you clarify the question a bit? Are you talking about looking into a projector beam? If that's the case, you'll pretty much just be temporarily blinded.

Yeah that's what I mean. What about a very low intensity projector?

 

[pivoxa15]

Objects reflect light in all directions at once (typically). So you'll see the exact same thing whether you are 10 feet in front of a mirror or 10 feet behind where it was (minus seeing yourself, of course). But the path the photons take to get to your eye will be different.

It is simple enough to visualize if you draw a diagram of where the rays go.

I am asking whether I can see the light before it reflects off an object or screen. i.e. Seeing a ver low intensity projector directly.

 

[pivoxa15]

Just because the concept is called 'real' doesn't mean it actually is an image of you in reality. The Physics Classroom has a thorough tutorial on [url=[PLAIN]http://www.physicsclassroom.com/Class/refln/reflntoc.html[/URL].[/QUOTE]

The link doesn't work.

 

[Integral]

Why don't you just pick up an magnifing glass and look through it? Look at a distant object while the lens is greater then the focal length from your eye.

 

[jtbell]

Set up a converging lens and an object such that a sharp real image is produced on a screen. Get behind the screen, with the screen between you and the lens, fairly close to your eye but not so close that you can't focus on the back of the screen (say, to read something printed there).

Now, remove the screen. You should see an inverted image of the object, floating in the air between you and the lens. Actually, if the image is large, you'll see only part of it, the part that's framed by the circle of the lens. By moving your head around, you can see different parts of the image.

If you move your head towards the former location of the screen, the image apparently gets bigger (because you're getting closer to it), but at some point you get too close and can't focus on it with your eyes any more, so it becomes blurry. When your eye reaches the actual image location, it's as if you were smacking the (enlarged or reduced) object itself right into your eye, except less painful.

 

[Danger]

Fascinating, jt. Would each eye receive the same image, or would parallax hold true to produce a 3-d picture?

 

[russ_watters]

Why not just find a projector, turn it off, and look through the lens?

 

[Integral]

Why not just find a projector, turn it off, and look through the lens?

Because the lamp will blind you?

 

[Integral]

Fascinating, jt. Would each eye receive the same image, or would parallax hold true to produce a 3-d picture?

I don't think so. The image you are viewing is created by the single lens, so you only have that perspective to work with.

Using paralax you can locate the image in space.

 

[russ_watters]

Because the lamp will blind you?

Um, I don't think it will. Perhaps you should should reread what I wrote...

 

[Danger]

I was wondering about that the other way around. Projectors (at least the ones that I've seen) are light-tight. How could you see into one?

 

[pivoxa15]

Set up a converging lens and an object such that a sharp real image is produced on a screen. Get behind the screen, with the screen between you and the lens, fairly close to your eye but not so close that you can't focus on the back of the screen (say, to read something printed there).

Now, remove the screen. You should see an inverted image of the object, floating in the air between you and the lens. Actually, if the image is large, you'll see only part of it, the part that's framed by the circle of the lens. By moving your head around, you can see different parts of the image.

If you move your head towards the former location of the screen, the image apparently gets bigger (because you're getting closer to it), but at some point you get too close and can't focus on it with your eyes any more, so it becomes blurry. When your eye reaches the actual image location, it's as if you were smacking the (enlarged or reduced) object itself right into your eye, except less painful.

I want to try to see it for myself but don't seem to have the equipment.

 

[pivoxa15]

Why don't you just pick up an magnifing glass and look through it? Look at a distant object while the lens is greater then the focal length from your eye.

I did that and I saw an inverted image which looked smaller than it really is.

Is that pretty much the same thing as jtbell was talking about? But I don't feel as if the image is hitting my eyes.

 

[Danger]

But I don't feel as if the image is hitting my eyes.

At the risk of sounding facetious... if it isn't hitting your eye, you can't see it.

 

[russ_watters]

I was wondering about that the other way around. Projectors (at least the ones that I've seen) are light-tight. How could you see into one?

Sorry, I left out a word there. That should be overhead projector.

 

[pivoxa15]

I was wondering about that the other way around. Projectors (at least the ones that I've seen) are light-tight. How could you see into one?

What kind of projectors are you talking about?

 

[pivoxa15]

At the risk of sounding facetious... if it isn't hitting your eye, you can't see it.

When using the magnifying glasses I didn't get this effect as talked by jtbell

"it's as if you were smacking the (enlarged or reduced) object itself right into your eye, except less painful."

I assume this does not occur for every image I see. The image from the magnifying glass looked like a normal image not the way jtbell would have described.

 

[jtbell]

I did that and I saw an inverted image which looked smaller than it really is.

Is that pretty much the same thing as jtbell was talking about? But I don't feel as if the image is hitting my eyes.

That's exactly it. When I do this in lab or as a demonstration, I usually have the object closer to the lens than the focal length so the image is enlarged. But if the object is farther away than the focal length, the image is reduced in size. That way is probably better for demonstration purposes... I need to remember to set it up that way next time.

To confirm that the image really is floating in front of the lens (from your point of view), you can place an object like a pencil between you and the lens, and move it back and forth. At a certain position, the image will appear to be stuck on the object so that they appear to move together when you move your head from side to side. This is the image position, and if you put a screen there you should see a sharply-focused image on it.

In the lab we have optical benches for this, so I just stick a pencil in a holder, in front of the lens, and slide it back and forth along the bench.

The image from the magnifying glass looked like a normal image not the way jtbell would have described.

If the image is sharp and upright it's a virtual image behind the lens, created by an object that is closer to the lens than the focal length. If the image is inverted it's a real image in front of the lens, and it should behave more or less as I described. If you move your eye closer to the lens, past the image position, the inverted image should disappear because it would now have to be behind your eye. At this point you'll see a blurred upright "image" which isn't really an image.

 

[jtbell]

Fascinating, jt. Would each eye receive the same image, or would parallax hold true to produce a 3-d picture?

If light coming from the object and passing through the lens can reach both of your eyes, you get the same 3-D "quality" from parallax that you would with a real object.

There's a little gadget that illustrates this vividly. It's made of two concave mirrors, glued together around their edges into a sort of "flying saucer" (classic UFO) shape. A circular opening is cut out of the center of one of the mirrors so you can see inside. Set the gadget down on a table with the hole facing upward, and put a coin (or other small object) inside it, resting at the center of the bottom mirror. If you look into the hole at an angle far enough away from the vertical that the object itself is hidden, all you see is a real image with the same size as the object, floating in the center of the hole. I've seen people reach into the hole to grab it, then gasp when their fingers go right through it.

 

[Danger]

Oh, man! I almost bought one of those things 35 years ago. Edmund Scientific sold them for about $20. One of the suggested uses was for the display of very expensive jewelry. If someone tried to grab it, it wasn't there. Having never studied optics, I could never figure out how the damned thing worked. Thanks for clearing that up!

 

[jtbell]

Now that you've jogged my memory about that thing, I'll try to remember to measure it when I'm on campus tomorrow, then figure out the radius of curvature of the mirrors and all the object and image distances. It's got to be a double reflection process. Light from the object first reflects off the upper surface of the cavity, forming image #1 (which is probably virtual). Then it reflects off the lower surface, forming image #2, the real image that you actually see.

 

[Danger]

Well, you jogged your own memory there. Don't blame it on me.

 

[pivoxa15]

To confirm that the image really is floating in front of the lens (from your point of view), you can place an object like a pencil between you and the lens, and move it back and forth. At a certain position, the image will appear to be stuck on the object so that they appear to move together when you move your head from side to side. This is the image position, and if you put a screen there you should see a sharply-focused image on it.

In the lab we have optical benches for this, so I just stick a pencil in a holder, in front of the lens, and slide it back and forth along the bench.

Am I meant to see the image reflect off the pencil? When I place a pencil between the magnifying glass and my eye, I don't see anything on the pencil, if anything it blocks some parts of the image that my eye is receiving when without the pencil.

Do I have to move my eye to the other side of the pencil (between the magnifying glass and pencil)?

The attachment is a diagram of my situation. Is it the same as what you are thinking?

If you move your eye closer to the lens, past the image position, the inverted image should disappear because it would now have to be behind your eye. At this point you'll see a blurred upright "image" which isn't really an image.

I see what you describe.

 

[Integral]

No, you are not looking for an image on the pencil. You are using parallax to locate the image. Hold the pencil between your eye and the lens, move your head side to side, you should be able to observe relative motion between the image and the pencil. Move back and forth between your eye and the lens until you find a spot where the image and pencil have no relative motion. The pencil will then be in the same distance from the lens as the image. If you replace the pencil with a screen you should see the image on the screen.

 

[EmilK]

The link doesn't work.

The link is http://www.physicsclassroom.com/Class/refln/reflntoc.html

 

[disregardthat]

Sorry, wrong place...