Re: virtual images and convex lenses

[Michael Porter <mbt@SYMPATICO.CA>]

On 1/5/02 1:17 PM, John S. Denker wrote:

> 2) The human eyeball is not and cannot be a suitable instrument
> for "seeing" real images, let alone virtual images.  A real image
> is formed upon the retina, but only after the light has been
> subjected to +25 diopters (or more) of intraocular refraction.
...

The fact that we use a real image on the retina to perceive both real and
virtual images is a potential source of confusion to students. However, I'm
not sure what you mean by your first sentence. Do you mean that the human
eye cannot distinguish between real and virtual images because it's always
trying to form a real image on the retina?

> Again, the eye is unsuitable for "seeing" real images.
> If you want to form a seeable real image, you need to build
> a camera obscura...
...
> There will be a real image hanging in space about one f
> in front of the lens, but you have no easly way of knowing
> that unless you interpose a screen.

It's been a while since I've played with this but I think I disagree, unless
you are saying that you could not detect the real image if you were standing
off to one side. If your eye is colinear with the object and the lens, then
the real image will be seen without the aid of a screen. To determine its
location, I've had the students focus on the image, then hold their thumb up
along the line of sight and move it back and forth until the thumb is in
focus. The location of the thumb ends up being roughly the same distance
from the lens as the image, verifiable by calculation. It's a nice exercise.

We have done a similar thing to locate the virtual image formed by a mirror
(plane, concave or convex, take your pick). I have the students focus on the
tip of a finger, and then I tell them to move it as close to their eye as
possible and still keep it in focus. This tells them the near limit of their
ability to focus. Then I place the mirror between their eye and their finger
(i.e. closer than they should be able to focus). They have no trouble
focusing on their image in the mirror, suggesting that the image is farther
away (behind) the mirror.

Mike
--
Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada