Binoculars
Last updated: April 25, 2008.
What
if you could walk on the Moon or stare an elephant right in the
eye? Binoculars and telescopes are the next best thing. They take you
right up to the action without having to move a muscle.
Binoculars are based on the science of optics and some pretty clever
tricks that lenses pull on light. But how exactly do binoculars zoom you from your
armchair to the middle of the solar system? Let's find out...
Photo: A sailor in the US Navy keeps a lookout
using binoculars
from the bridge of an aircraft carrier.
Public domain photo courtesy
of US Navy.
How lenses work
The way light bends when it enters water (or any other more dense
material, such as glass) is called refraction.
(For a full explanation of how refraction works, please see our more detailed article on
light.)
Refraction is
the key to how lenses work—and lenses are
the key to
binoculars, telescopes, and glasses. But how do we get from a stick
that bends in water to a cool pair of binoculars that let us study the
moon?
Water sitting in a glass appears to have a straight upper edge, even
though it is very slightly curved (the curved edge has a special name:
it's called a meniscus). If you place a
glass on top of a
newspaper and look straight down, the news print looks just the same as
normal. That's because the top of the water is effectively straight.
But if the water had a curved upper surface, the news print would look
magnified. You can see this for yourself by trying the simple activity
below.
Types of lens
A lens is a curved piece of glass shaped a bit like a lentil. (If
you ever wondered where a lens gets it name from, that's where: lens
comes from the Latin word for lentil.) When light rays hit a glass
lens, they slow down and bend. If the lens curves like a lentil (like a
dome), so its outside is thinner than its middle, it's called a convex
lens. As light rays enter a convex lens, they bend in toward the
middle—as though the lens is sucking them in. That means a convex lens
brings distant light rays into a focus. It's also called a converging
lens because it makes light rays come together (converge).
Looking
at things through a convex lenses makes them appear bigger—so convex
lenses are used in things like magnifying glasses.
Another kind of lens curves the opposite way, with the middle
thinner than the outside. This is called a concave
lens. (You
can remember this easily if you think that a concave lens caves in
in the middle.) A concave lens makes light rays spread out like the
lines in a firework. Imagine light rays coming into a concave lens and
then shooting out in all directions. That's why a concave lens is
sometimes called a diverging lens. It makes
light rays shoot
out (diverge). Concave lenses are used in movie projectors to make
light from the film spread out and cover a bigger area when it hits the
wall.
How binoculars work
You can probably see where we're heading. If you want to see
something in the distance, you can use two convex lenses, placed one in
front of the other. The first lens catches light rays from the distant
object and makes a focused image a short distance behind the lens. This
lens is called the objective, because it's
nearest to the
object you're looking at. The second lens picks up that image and
magnifies it, just like a magnifying glass magnifies an image on paper.
If you put the two lenses in a closed tube, hey presto, you have a
telescope. There's quite a good demonstration on this page at Birdwatching.com.
You can make your own telescope easily enough with a couple of
magnifying glasses and a cardboard tube wrapped around them.
Binoculars are simply two telescopes side by side, one for each eye.
But there's one catch. When light rays from a distant object pass
through a convex lens, they cross over. That's why distant things
sometimes look upside down if you look at them through a magnifying
glass. The second lens doesn't sort out that problem. So binoculars
have a pair of prisms (large wedges of
glass) inside them to rotate the
image through 180 degrees. One prism rotates the image through 90
degrees (flips it onto its side), then the next prism rotates it
through another 90 degrees (flips it onto its side again), so the two
prisms effectively turn it upside down. The prisms can either be
arranged in a back-to-back arrangement (known as roof
prisms) or at 90
degrees (known as Porro prisms).
The prisms explain why binoculars are heavy and why they are
sometimes quite chunky in the middle. Field glasses, which are compact
binoculars like the ones shown in the photo here, flip the incoming
images using only lenses. There are no prisms, so field glasses are
smaller, lighter and more compact—but the image quality is poorer.
Photo: Key features of field glasses. You focus
by turning the focusing screw in the middle. This pushes the focusing
mechanism back and forward, increasing the distance between the
objective lens and the eyepiece lens.
