Re: [Phys-l] Image of object placed at focal point

[John Denker <jsd@av8n.com>]

Folkerts, Timothy J wrote:

> The eyepiece of a microscope (or telescope or other similar instrument)
> is often adjusted so that its object (the image of the objective lens)
> is approximately at the focal point.  This creates a virtual image "far
> away" so that the person looking through the instrument can relax
> his/her eye.  
>
> That sounds pretty common and pretty useful to me!

An excellent point.  I never thought of that as a virtual image,
but of course it is.

Heads-up displays are another category of very useful virtual
images.  HUDs are found in aircraft.  A variant you may be more
likely to encounter is a Telrad (wonderful for aiming a telescope)
(although nowadays you can do just as well using a bright DPSS
laser).

Part of the trick is using a two-way mirror to make it easy to
see the virtual image.