Telescopes.

[Gareth56]

Looking through my telescope (refracting) I note that right & left are reversed, why does a telescope do this and not binoculars?

[Pangloss]

If I remember correctly, the prism in binoculars reverses the image back to normal. This isn't done in a telescope because it causes loss of light -- you want as few elements in the way as possible because the objects are so dim. But with binoculars you're typically looking at daylight objects so loss of light isn't such a big deal.

 

We used to call that "perverted", by the way, when the image is reversed left-to-right. As opposed to "inverted" for up-down reversal. Haven't heard it used in years, though -- it always caused a Beavis & Butthead kind of chuckle even amongst astronomers anyway.

[Gareth56]

So we're still no wiser as to why a telescope reverses an image.

[Klaynos]

It depends on the design of the telescope, but assuming it's a normal 2 lens refracting telescope, if you apply simple ray tracing you can see that there is a focal point between the two lenses so the light literally crosses over itself, let me find an image....

 

http://en.wikipedia.org/wiki/Image:Telescope-schematic.png

 

There you go.

[Jacques]

First not all telescopes revert the image: The galilean telescope doesn't invert the image.

It is made with a convex objective lens and a concave ocular lens. That kind of telescope is very limited in magnification and are sometime used in theater binocular, no need for heavy prisms to revert the image back.

But for more powerfull scope the occular is also a convex lens. If do some raytracing, you will understand why the image is inverted. raytracing applet

Most of the time, refracting telescope have a little mirror or prism before the objective that revert back the up and down but not the left and right.

[Gareth56]

I understand the inverting aspect but not why refracting telescopes reverse left and right as when I look at the srface of the Moon the dark maria appear on the left when looking through the scope but when I look with my eyes they are on the right hand side.

[Klaynos]

left/right is just the same as up/down inversion...

 

Your scope will be circularly symmetric...