though a lens before hitting the curved
mirror. The lens helps to make the
image sharper.
Some light telescopes are fairly small
tubes that sit on a stand and can be carried
around easily. Buildings called
observatories house much larger, more
powerful light telescopes. Spacecraft may
also carry light telescopes. The Hubble
Space Telescope, which orbits Earth on a
spacecraft, is a type of reflecting telescope.
Other Telescopes
Some types of telescopes do not collect
light. These telescopes collect other
forms of energy from space—for
example, radio waves, infrared radiation
(a type of heat), and X-rays. Planets,
stars, gas, and other things in space give
off these types of energy.
Radio telescopes look like huge bowls.
They collect radio waves that travel to
Earth’s surface. Infrared, X-ray, and
other similar telescopes are mounted on
spacecraft.
All these telescopes allow scientists to
gather information about things in space
that cannot necessarily be seen. For
example, they have shown that there is
water vapor in other parts of the Milky
Way galaxy. They have also helped scientists
understand how stars and planets
form and how stars die.
History
By the early 1600s several people had
made simple refracting telescopes. In
1609 the Italian scientist Galileo
improved the design of a telescope made
by a Dutch inventor. Later in the 1600s
several inventors, including the English
scientist Isaac Newton, built reflecting
telescopes.
A U.S. astronomer built the first radio
telescope in 1937. Other scientific telescopes
developed along with spacecraft,
which scientists began launching in the
1950s.
#More to explore
Astronomy • Galileo • Lens • Light
• Mirror
Television
Television, or TV, is a system for sending
moving pictures and sound from one
place to another. It is one of the most
important and popular forms of communication.
TV programs provide news,
information, and entertainment to
people all over the world.
The Very Large Array is a group of 27
bowl-shaped radio antennas in the state of
New Mexico. Each antenna is 82 feet (25
meters) across. When used together they
make one very powerful radio telescope.
BRITANNICA STUDENT ENCYCLOPEDIA Television 39
How TVWorks
TV begins with a television camera, or
video camera. The camera records the
pictures and sound of a TV program. It
changes the pictures and sound into
electric signals. A TV set receives the
signals and turns them back into pictures
and sound.
The TV Signal
A standard TV camera changes the pictures
into an electric signal called the
video signal. The video signal carries the
pictures in the form of tiny dots called
pixels. The camera’s microphone
changes the sound into another electric
signal, called the audio signal. The video
and audio signals together form the TV
signal.
Digital TV, or DTV, is a newer way of
handling TV signals. A digital TV signal
carries pictures and sound as a number
code, like a computer does. A digital
signal can carry more information than a
standard signal can, which creates better
pictures and sound. High definition TV,
or HDTV, is a high-quality form of
digital TV.
People who own plasma TVs often hang
them on a wall. Plasma TVs are thinner and
lighter than standard TVs.
Television stations may broadcast, or send, TV signals with antennas, satellites, or cables.
The signals sent with antennas may reach houses directly.
40 Television BRITANNICA STUDENT ENCYCLOPEDIA
A TV signal can reach a TV set in several
ways. Local TV stations use antennas to
send, or broadcast, signals through the
air as radio waves. Cable TV stations
send signals through underground
cables. Satellites, or spacecraft, traveling
high above Earth can send signals to
special antennas called satellite dishes. A
signal can also come from a VCR, DVD
player, or DVR (digital video recorder)
connected to the TV set. VCRs, DVRs,
and some DVD players can record a TV
signal coming into the TV and then play
it back later.
Display
A standard TV set turns the video signal
into beams of tiny particles called electrons.
It shoots these beams at the back
of the screen through a picture tube.
The beams “paint” the pixels on the
screen in a series of rows to form the
picture. The TV set sends the audio
signal to loudspeakers.
LCD and plasma TVs form the picture
differently. They do not use a picture
tube and electron beams. Because they
do not hold a picture tube, LCD and
plasma TVs are much thinner and
lighter than standard TVs. They can
even hang on a wall.
LCD stands for liquid crystal display.
Liquid crystal is a substance that flows
like a liquid but has some tiny solid
parts, too. The display sends light and
electric current through the liquid crystal.
The electric current causes the solid
parts to move around. They block or let
light through in a certain way to make
the picture on the screen.
A plasma display has many tiny cells, or
units, containing gas. Electricity sent
through the gas forms a plasma. A
plasma is a bunch of particles that have
an electric charge. The plasma gives off
light, which makes the picture.
