Print ThisHow Do Binoculars Work?
It's a lot easier to carry binoculars on a hike than a telescope.
by Bart Coenders
A Bit of History
If you’ve read the page on telescopes, then you already know a lot about how binoculars work. Binoculars are really just two refracting telescopes held side-by-side, with a few extra tricks thrown in.
The first binoculars were generally too long and clumsy to work very well. In 1854, Italian inventor Ignatio Porro came up with a brilliant idea. Instead of passing the light straight through the tubes from end to end, Porro realized he could make the tubes much shorter by bouncing the light around inside.
Porro’s design, which is still used today in many binoculars, uses two prisms to change the direction of the light beams inside the binocular tubes. The prisms also flip the image so that, unlike in a normal telescope, the image doesn’t end up upside-down when it reaches your eyes.
How It Works
Light enters the front and bounces off two prisms before passing through the eyepiece lens and entering your eye.
by David Garrison
Binoculars are great for seeing things on Earth and in the sky. Suppose you see a strange light on the horizon. Have the flying saucers finally arrived from Alpha Centauri? You grab your binoculars and take a look. When you do, the light from the UFO encounters the objective lenses (one on each side) of the binoculars. Just as in a telescope or a microscope, the glass lens bends the light by slowing it down. The bent light beams race through the body of the binoculars, bouncing off the two prisms before they pass through the eyepiece and enter your eyes.
The size and placement of the lenses determine just how magnified the image is when it enters your eyes. For instance, binoculars rated 7 x 50 make the image seven times larger. The 50 is the size of the objective lens, measured in millimetres. The larger the objective lens, the more light it gathers from the object. These binoculars are just strong enough to make out some strange inscriptions on the UFO: “Goodyear”. The unidentified flying object has just become an identified flying object. Oh, well!
Food For Thought
Focus your binoculars on the Pleiades star cluster for a celestial treat.
by Paul LeFevre
Binoculars are great instruments for viewing objects here on Earth, but they’re also great tools for exploring the universe beyond. One of the best objects to view with binoculars is a cluster of stars called the Pleiades [say Plee-ah-dees]. The Pleiades are especially easy to find in the winter sky. Find this cluster on a star chart and take a look. What you’re seeing is a group of “newly-born” stars (less than 100 million years old!) that are around 440 light-years from Earth.
Think about what that means: this light left the Pleiades 440 years ago, during Galileo’s time. That light finally reaches the Earth, 440 years later, passes through your binoculars, and reaches your retina. And you get to see one of the most beautiful sights in the night sky. How cool is that?
For Further Information
Binocular Highlights: 99 Celestial Sights for Binocular Users by Gary Sironek (2007). This book explains how to use a pair of binoculars to begin your exploration of the universe.
Touring the Universe through Binoculars: A Complete Astronomer's Guidebook by Philip S. Harrington (1990). A guide that includes over 1100 deep sky objects you can see with binoculars including globular clusters, double stars, binary star systems, and nebulae.