View Full Version : How Lenses work

2011-Apr-03, 02:46 PM
I understand that in a refracting telescope, the primary lens forms an inverted image at the focal point. Is this inverted image subsequently turned right side up at the eye piece?
Also, why don't I see an inverted image when I look through a magnifying glass?

2011-Apr-03, 05:37 PM
Telescopes form an inverted image in all cases as does your magnifying glass! The objective lens forms an image that is upside down, the eyepiece just magnifies this so it stays upside down. Binoculars use special prisms to turn the image upright. This lose some light so are not used in astronomical telescopes.

Use the magnifying glass as your objective and it does the same. Or use it to focus an image of a bright window on a piece of paper and you'll see it's image upside down on the paper. Now hold the magnifying glass well away from your eye and look at the room, it will be upside down.

When you use it as a magnifying glass you are well inside the focus before the image has a chance to be inverted. While holding the magnifying glass so the room is upside down, walk toward some object, as you get closer and closer it will start to get fuzzy and finally vanish into a total blur. You are now at the focus point where an eyepiece is needed to see a magnified image. Keep moving in and it will start to clear up but now will be right side up!


2011-Apr-04, 03:07 PM
Thanks. Fully explained, and without a single formula!