How Binoculars Work
Essentially, a pair of binoculars is simply two telescopes mounted side by side. One for each eye. To understand binoculars, you need to understand how a telescope works.
Here's an easy demonstration that you can try yourself. All you need are two ordinary magnifying glasses and a piece of tracing paper. Do this once, and you will understand forever how binoculars work.
Set up a light bulb and turn it on. (Any brightly lit object will do.) Hold a magnifying glass a few feet from the bulb. Then dangle the tracing paper on the other side, so that the light shines through the magnifying glass onto the tracing paper. (It helps to have an assistant hold the paper.)
Move the paper back and forth. At a certain distance from the magnifying glass, a small, upside-down-and-backwards image of the light bulb will form on the paper.
The magnifying glass is acting the part of the telescope's objective lens, where light enters the scope. The light bulb is the object being observed through the telescope.
Want to enlarge the image on the tracing paper? Use a second magnifying glass, and look at the image on the paper. That second magnifying glass acts the part of the eyepiece of the telescope (the part you look through).
Here's the surprising part. Keep looking at the image on the tracing paper, and while you're doing that, have your assistant take the tracing paper away.
The image remains! In fact, it will appear even brighter and clearer than with the paper. You don't actually need the tracing paper to see the image when you look at it through the second magnifying glass.
That's a telescope!
You have just made a working telescope! You could use the two magnifying glasses to magnify any distant object. Just line up the two magnifying glasses, hold them at two distances from your eye, and look through both. Whatever you see through the two lenses will be enlarged. Put a tube around them, and you have a telescope.
And in fact, this is how the first telescope was invented.)
But it's upside down!
The only problem with this elementary telescope is that it makes everything look upside down and backwards. That would be OK for looking at stars. But for watching birds or wildlife, we require a right-side-up picture. A terrestrial telescope (used to look at objects on earth rather than in the heavens) has show things oriented correctly.
To correct the orientation, a telescope needs a third element, the erecting prisms.
A prism is a solid piece of glass that functions as a mirror, but without a mirror's reflective backing. Light rays that have entered a prism cannot get out if they strike a surface at too great an angle. Instead, they reflect back, as if from a perfect mirror.
In the mid 19th Century, an Italian named Porro designed a telescope with two prisms set at right angles to each other between the objective lens and the eyepiece. This arrangement not only erected and reversed the image, but also folded the light path, resulting in a shorter, more manageable telescope.
Prior to Porro's invention, it was not practical to put two telescopes together for viewing through both eyes. The tubes were simply too long and unwieldy.
Porro's creation made binoculars possible. Then, in 1894, the Zeiss Optical Works created the first "Hunting Glasses," incorporating the Porro prism design. Modern binoculars were born.
Since then, binoculars have improved enormously. However the essential elements are the same today.