Real and Virtual images

I've been investigating how the optics in a telescope work, and whilst I
understand the relationships behind focal lengths, aperture, magnification,
and f ratio, I have been driving myself insane trying to work out WHY a
telescope works the way it does!

To try out a few things, I've started at the beginning with a magnifying
glass (a simple convex lens). From this, I can understand the differences in
the images you see of objects which are closer to the glass than its focal
length (i.e. they are magnified), objects *at* the focal length
(theoretically maximum magnification, but usually blurred and distorted due
to the properties of light and imperfections in the glass), and objects more
distant than the focal length (they are smaller, and upside down)

By drawing out ray trace diagrams, I can see why these effects happen. In
all of these examples, the eye is viewing a virtual image (i.e. it's looking
'through' the glass) - It is the diverging rays coming out of the lens which
are being focussed onto the retina by the lens in my eye.

I am also aware that at my side of the glass, there is a 'real' image being
formed, floating at the focal plane. I've proved this by putting some
tracing paper at the focal point to allow me to see the real image.

Now, my question.... If I bring my eye to this focal point (where the
tracing paper was), is it possible for my eye to see the real image? - Does
this real image get projected onto my retina in a similar way it was
displayed on the tracing paper? But what happens to the 'virtual image'
that's also hitting my retina, don't the two interfere with each other
somehow?

John.

John Honan wrote:
By drawing out ray trace diagrams, I can see why these effects happen. In
all of these examples, the eye is viewing a virtual image (i.e. it's looking
'through' the glass) - It is the diverging rays coming out of the lens which
are being focussed onto the retina by the lens in my eye.

I am also aware that at my side of the glass, there is a 'real' image being
formed, floating at the focal plane. I've proved this by putting some
tracing paper at the focal point to allow me to see the real image.

Now, my question.... If I bring my eye to this focal point (where the
tracing paper was), is it possible for my eye to see the real image? - Does
this real image get projected onto my retina in a similar way it was
displayed on the tracing paper? But what happens to the 'virtual image'
that's also hitting my retina, don't the two interfere with each other
somehow?

If there's nothing between you and the real image, there is no virtual
image for you to see. You see the real image. However, you have to be
a few inches back from the real image for your eye to be able to focus
on it. Otherwise, there's just a blur of light.

With a single lens, if the lens is closer to the object than the focal
length, a virtual image is formed. If it's further away, a real image
is formed somewhere between you and the lens. Precisely at the focal
length, there is no image formed anywhere (or, equivalently, the image
is formed at infinity).

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"Brian Tung" wrote in message
...
If there's nothing between you and the real image, there is no virtual
image for you to see. You see the real image. However, you have to be
a few inches back from the real image for your eye to be able to focus
on it. Otherwise, there's just a blur of light.

With a single lens, if the lens is closer to the object than the focal
length, a virtual image is formed. If it's further away, a real image
is formed somewhere between you and the lens. Precisely at the focal
length, there is no image formed anywhere (or, equivalently, the image
is formed at infinity).

Thanks for the explanation Brian. I thought the real image had to be formed
on something (hence the reason for putting tracing paper at the focal
point) - If the tracing paper isn't on the focal point, the real image will
appear out of focus. So how can you see the real image just floating there
if your eye is not positioned at (or near) the focal point?

So what happens as you move the lens further away from the object is this
(please correct me if I'm wrong);

1) Up until the focal point distance, you are seeing a 'right side up' image
that gradually gets bigger as the object approaches the focal distance. You
are seeing the virtual image. There is no real image.

2) As you continue past the focal distance the image becomes inverted. You
are now seeing the real image. There is no virtual image.

Does this sound right? - A real OR a virtual image can be seen, depending on
the distance of the object and the focal length. But never both.

John Honan wrote:
Thanks for the explanation Brian. I thought the real image had to be formed
on something (hence the reason for putting tracing paper at the focal
point) - If the tracing paper isn't on the focal point, the real image will
appear out of focus. So how can you see the real image just floating there
if your eye is not positioned at (or near) the focal point?

Light rays converge at the real image. However, they don't stop there.
They go right through the real image (since there's nothing physical
there), and diverge from it--just as they diverge from an actual physical
object. That's why it's called a real image, I think--because light
rays are emitted from (or, more precisely, through) it almost as though
it were a real thing.

So, just as you can't see an object clearly if your eye is half an inch
from it, you can't see a real image if your eye is half an inch from it.
You have to back up further before your eyes can focus the divergent
light rays onto your retina.

So what happens as you move the lens further away from the object is this
(please correct me if I'm wrong);

1) Up until the focal point distance, you are seeing a 'right side up' image
that gradually gets bigger as the object approaches the focal distance. You
are seeing the virtual image. There is no real image.

Right, and as you approach the focal length, that virtual image appears
further and further in direct proportion to its apparent linear size.
Its *angular* size therefore approaches a constant limit.

2) As you continue past the focal distance the image becomes inverted. You
are now seeing the real image. There is no virtual image.

Provided your eye is further from the lens than the real image, then
yes, you see the inverted real image. Between the lens and the real
image, your eye will just see a blur.

Does this sound right? - A real OR a virtual image can be seen, depending on
the distance of the object and the focal length. But never both.

Essentially, yes. There is some play in the actual distances involved,
since your eye is capable of accommodating--that is, of adjusting its
focus in response to different objects.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt