Transcript The Electromagnetic Spectrum

Objectives
• Describe what waves and more specifically,
electromagnetic waves are and how they are
produced.
• Recognize that electricity and magnetism are two
aspects of a single electromagnetic force.
• Explain how electromagnetic waves transfer energy.
• Describe various applications of electromagnetic
waves.
Waves
Direction
A wave is a pattern that repeats itself in a cycle of both
time and space. Waves are characterized by the velocity
with which they move, their frequency, and their
wavelength.
Waves
wave period
• The amount of time required for a wave to repeat
itself at a specific point in space
frequency
• The number of wave crests passing any given point
per unit of time.
Waves
wavelength
The length from one point on a wave to the point where
it is repeated exactly in space, at a given time.
The wave speed is simply the product of the wavelength
and the frequency:
Waves
diffraction
The tendency of waves to bend around corners. The
diffraction of light establishes its nature as a wave.
interference
The ability of two or more waves to interact in such a
way that they either reinforce or cancel each other.
Waves
• If radiation were composed
of rays or particles moving
in perfectly straight lines,
no bending would occur
• the outline of the hole
becomes "fuzzy" due to
bending from diffraction
Waves
Propagation of Electromagnetic Waves
• Electromagnetic waves are comprised of oscillating,
perpendicular electric and magnetic fields.
• They travel at the speed of light.
• Electromagnetic waves are transverse waves; that is,
the direction of travel is perpendicular to the the
direction of oscillating electric and magnetic fields.
Electromagnetic Waves
Propagation of Electromagnetic Waves
• All electromagnetic waves are produced by
accelerating charges.
• Electromagnetic waves transfer energy. The energy
of electromagnetic waves is stored in the waves’
oscillating electric and magnetic fields.
• Electromagnetic radiation is the transfer of energy
associated with an electric and magnetic field.
Electromagnetic radiation varies periodically and
travels at the speed of light. It can be in a vacuum.
The Sun at Different Wavelengths of Radiation
Propagation of Electromagnetic Waves
• High-energy electromagnetic waves behave like
particles.
• An electromagnetic wave’s frequency makes the
wave behave more like a particle. This notion is
called the wave-particle duality.
• A photon is a unit of light.
• Photons can be thought of as particles of
electromagnetic radiation that have zero mass and
carry one quantum of energy.
The Electromagnetic Spectrum
• The electromagnetic spectrum ranges from very long
radio waves to very short-wavelength gamma waves.
• The electromagnetic spectrum has a wide variety of
applications and characteristics that cover a broad
range of wavelengths and frequencies.
The Electromagnetic Spectrum
• Radio Waves
– longest wavelengths
– communications, tv
• Microwaves
– 30 cm to 1 mm
– radar, cell phones
• Infrared
– 1 mm to 700 nm
– heat, photography
• Visible light
– 700 nm (red) to 400 nm
(violet)
• Ultraviolet
– 400 nm to 60 nm
– disinfection,
spectroscopy
• X rays
– 60 nm to 10–4 nm
– medicine, astronomy,
security screening
• Gamma Rays
– less than 0.1 nm
– cancer treatment,
astronomy
The Electromagnetic Spectrum
The Electromagnetic Spectrum
visible spectrum
• The small range of the electromagnetic spectrum that
human eyes perceive as light. The visible spectrum
ranges from about 4000 to 7000 angstroms,
corresponding to blue through red light.
The Electromagnetic Spectrum
Note that wave
frequency (in hertz)
increases from left to
right, and wavelength
(in meters) increases
from right to left. These
wave properties
behave in opposite
ways because, as
noted earlier, they are
inversely related.
The Electromagnetic Spectrum
• Our eyes are sensitive to only a minute portion of
the many different kinds of radiation known.
• Only a small fraction of the radiation produced by
astronomical objects actually reaches our eyes,
in part because of the opacity of Earth's
atmosphere.
The Electromagnetic Spectrum
• Opacity is the extent to which radiation is blocked
by the material through which it is passing—in
this case, air.
• The more opaque an object is, the less radiation
gets through it.
• Opacity is the opposite of transparency.
The Electromagnetic Spectrum
The Electromagnetic Spectrum
• What causes opacity to vary along the spectrum?
Certain atmospheric gases are known to absorb
radiation very efficiently at some wavelengths.
For example, water vapor (H2O) and oxygen (O2)
absorb radio waves having wavelengths less than
about a centimeter, whereas water vapor and
carbon dioxide (CO2) are strong absorbers of
infrared radiation.
Ultraviolet, X-ray, and gamma-ray radiation are
completely blocked by the ozone layer high in
Earth's atmosphere.
The Electromagnetic Spectrum
• White light is a mixture of colors, which we
conventionally divide into six major hues—red,
orange, yellow, green, blue, and violet.
• In principle, the original beam of white light could
be restored by passing the entire red-to-violet
range of colors—called a spectrum—through a
second, oppositely oriented prism to recombine
the colored beams.
• This experiment was first reported by Isaac
Newton over 300 years ago.
The Electromagnetic Spectrum
• While passing through a prism, white light splits into its
component colors, spanning red to violet in the visible part
of the electromagnetic spectrum. The slit narrows the beam
of radiation. The image on the screen is just a series of
different-colored images of the slit.
The Electromagnetic Spectrum
• Astronomers often use a unit called the
nanometer (nm) when describing the wavelength
of light (see Appendix 2).
• There are 109 nanometers in 1 meter. An older
unit called the angstrom (1Å - 10 -10 m - 0.1 nm)
is also widely used.
• The unit is named after the nineteenth-century
Swedish physicist Anders Ångstrom—
pronounced "ongstrem.“
• In SI units, the nanometer is preferred. Thus, the
visible spectrum covers the wavelength range
from 400 to 700 nm (4000 to 7000 Å).
Blackbody Radiation
intensity
A basic property of electromagnetic radiation that
specifies the amount or strength of the radiation.
Often used to specify the amount or strength of
radiation at any point in space.
Blackbody Radiation
•No natural object emits all its radiation at just one
frequency.
•Energy is generally spread out over a range of
frequencies.
•We look at the way in which the intensity of this
radiation is distributed across the electromagnetic
spectrum,
Blackbody Radiation
The blackbody, or Planck, curve represents the
distribution of the intensity of radiation emitted by
any heated object.
Blackbody Radiation
blackbody curve
The characteristic way in which the intensity of
radiation emitted by a hot object depends on
frequency.
The frequency at which the emitted intensity is
highest is an indication of the temperature of the
radiating object.
Also referred to as the Planck curve.
Blackbody Radiation
As an object is
heated the
radiation it emits
peaks at higher
and higher
frequencies.
Shown here are curves corresponding to temperatures of 300 K
(room temperature), 1000 K (beginning to glow deep red), 4000 K
(red hot), and 7000 K (white hot).
Blackbody Radiation
As the temperature continues to rise, the peak of the
metal's blackbody curve moves through the visible
spectrum, from red (the 4000 K curve) through
yellow.
The metal eventually becomes white hot when its
blackbody curve peaks in the blue or violet part of
the spectrum (the 7000 K curve),
However, the low-frequency tail of the curve extends
through the entire visible spectrum so white light is
emitted.
Blackbody Radiation
Many extraterrestrial objects, however, do emit
copious quantities of ultraviolet, X-ray, and even
gamma-ray radiation.
Although most sunlight is visible, a great deal of
information about our parent star can be obtained by
studying it in other parts of the electromagnetic
spectrum.
Blackbody Radiation
Four images of the Sun, made using (a) visible light, (b) ultraviolet
light, (c) X-rays, and (d) radio waves. By studying the similarities
and differences among these views of the same object,
astronomers can find important clues to its structure and
composition
Blackbody Radiation
Other cosmic objects have surfaces very much cooler or
hotter than the Sun's, emitting most of their radiation in
invisible parts of the spectrum
a) A cool, invisible galactic gas
cloud called Rho Ophiuchi. At a
temperature of 60 K, it emits mostly
low-frequency radio radiation.
(b) A dim, young star (shown here in
red) near the center of the Orion
Nebula. The star's atmosphere, at
600 K, radiates primarily in the
infrared.
c) The Sun's surface, at 6000 K, is
brightest in the visible region of the
electromagnetic spectrum.
d) A cluster of very bright stars, called Omega
Centauri, as observed by a telescope aboard a
space shuttle. At a temperature of 60,000 K,
these stars radiate strongly in the ultraviolet.
Radio Astronomy
radio telescope
Large instrument designed to detect radiation
from space in radio wavelengths.
Radio Astronomy
•The radio window in the electromagnetic
spectrum is much wider than the optical window..
• Atmosphere is no hindrance to long-wavelength
radiation, radio astronomers have built many
ground-based radio telescopes to detect cosmic
radio waves.
•These devices have all been constructed since
the 1950s—radio astronomy is a much younger
subject than optical astronomy.
Radio Astronomy
a fairly typical radio telescope, the large 43-m (140-foot)-diameter
telescope located at the National Radio Astronomy Observatory in
West Virginia.
much larger than reflecting optical telescopes, most radio
telescopes are built in basically the same way.
They have a large, horseshoe-shaped mount supporting a huge,
curved metal dish that serves as the collecting area.
The dish captures cosmic radio waves and reflects them to the
focus, where a receiver detects the signals and channels them to a
computer.
Radio Astronomy
However, unlike optical instruments, which can detect all
visible wavelengths simultaneously, radio detectors normally
register only a narrow band of wavelengths at any one time.
To observe radiation at
another radio
frequency, we must
retune the equipment,
much as we tune a
television set to a
different channel.
Radio Astronomy
An aerial photograph of the 300-m-diameter dish in Puerto Rico. The
receivers that detect the focused radiation are suspended nearly 300
m above the dish.
a close-up of
the radio
receivers
hanging high
above the dish.
Technicians adjusting the dish surface.
Radio Astronomy
Despite the inherent disadvantage of relatively poor
angular resolution, radio astronomy enjoys many
advantages:
•Radio telescopes can observe 24 hours a day.
•Observations can often be made through cloudy skies,
and they can detect the longest-wavelength radio waves
even during rain or snowstorms
•It opens up a whole new window on the universe
New Universe
1. First, just as objects that are bright in the
visible part of the spectrum (the Sun, for
example) are not necessarily strong radio
emitters, many of the strongest radio sources
in the universe emit little or no visible light.
2. Second, visible light may be strongly
absorbed by interstellar dust along the line of
sight to a source. Radio waves, on the other
hand, are generally unaffected by intervening
matter.
New Universe
The Orion Nebula is a starforming region about 1500
light years from Earth.
The bright regions in this
photograph are stars and
clouds of glowing gas. The
dark regions are not empty,
but their visible emission is
obscured by interstellar
matter.
Superimposed on the optical image is a radio contour map of the
same region. Each curve of the contour map represents a different
intensity of radio emission. The resolution of the optical image is
about 1"; that of the radio map 1'.
Interferometry
interferometer
Collection of two or more telescopes
working together as a team, observing
the same object at the same time and at
the same wavelength. The effective
diameter of an interferometer is equal to
the distance between its outermost
telescopes.
Interferometry
In interferometry, two or more radio telescopes are used
in tandem to observe the same object at the same
wavelength and at the same time
Through electronic cables or radio links, the signals
received by each antenna in the array making up the
interferometer are sent to a central computer that
combines and stores the data.
The technique works by analyzing how the waves
interfere with each other when added together.
Interferometry
This large interferometer is made up of 27 separate dishes
spread along a Y-shaped pattern about 30 km across on the
Plain of San Augustin, NM. The most sensitive radio device in
the world, it is called the Very Large Array or VLA, for short.
(b) A close-up view from ground level of some of the VLA
antennas. Notice that the dishes are mounted on railroad
tracks so that they can be repositioned easily.
Interferometry
VLA radio "radiograph” of the spiral galaxy M51, observed at radio
frequencies with an angular resolution of a few arc seconds (a) shows nearly
as much detail as (b) an actual (light) photograph of that same galaxy made
with the 4-m Kitt Peak optical telescope.
Interferometry
In 1997 a group of scientists in Cambridge, England,
succeeded in combining the light from three small
optical telescopes to produce a single, remarkably
clear, image.
Each telescope is only 0.4 m in diameter, but when the
equipment is positioned 6 m apart, the resulting
resolution is a stunning 0.01"
Infrared Telescopes
• Infrared studies are a very important component of
modern observational astronomy.
• Infrared telescopes resemble optical telescopes
(indeed, many optical telescopes are also used for
infrared work), but their detectors are sensitive to
longer-wavelength radiation.
Infrared Telescopes
•
Although most infrared radiation is absorbed by the
atmosphere (primarily by water vapor), there are a few
windows in the high-frequency part of the infrared spectrum
where the opacity is low enough to allow ground-based
observations
•
Some of the most useful infrared observing is done from the
ground, even though the radiation is somewhat diminished
in intensity by our atmosphere.
•
As with radio observations, the longer wavelength of infrared
radiation often enables us to perceive objects partially
hidden from optical view.
Infrared Telescopes
•
An optical photograph (a) taken near San Jose, California,
and an infrared photo (b) of the same area taken at the
same time.
•Longer-wavelength infrared radiation can
penetrate smog much better than shortwavelength visible light.
Infrared Telescopes
(a) The Orion Nebula and its surrounding environment was made by the Infrared
Astronomy Satellite. The whiter regions denote greater strength of infrared radiation;
the false colors denote different temperatures, descending from white to red to black.
(b) The same region photographed in visible light. The labels alpha and beta refer to
Betelgeuse and Rigel, the two brightest stars in the constellation. Note how the red star
Betelgeuse can be seen in the infrared (part a), but the blue star Rigel cannot.
UV Telescopes
ultraviolet telescope
A telescope that is designed to collect radiation in the ultraviolet
part of the spectrum. The Earth's atmosphere is partially
opaque to these wavelengths, so ultraviolet telescopes are
put on rockets, balloons, or satellites to get high above most
or all of the atmosphere.
The ultraviolet domain, this region of the spectrum, extends in
wavelength from 400 nm (blue light) down to a few
nanometers, has only recently begun to be explored.
UV Telescopes
Earth's atmosphere is partially opaque to
radiation below 400 nm and is totally opaque
below about 300 nm
Rockets, balloons, or satellites are therefore
essential
Hubble Space Telescope best known as an
optical telescope, is also a superb ultraviolet
instrument.
Telescopes
High-energy astronomy studies the universe as it
presents itself to us in X-rays and gamma rays—the
types of radiation whose photons have the highest
frequencies and hence the greatest energies.
How do we detect radiation of such short
wavelengths?
Telescopes
First, it must be captured high above Earth's
atmosphere because none of it reaches the ground.
Second, its detection requires the use of equipment
basically different in design from that used to
capture the relatively low energy radiation
discussed up to this point.
Telescopes
The basic difference in the design of high-energy telescopes comes about
because X and gamma rays cannot be reflected easily by any kind of
surface.
These rays tend to pass straight through, or be absorbed by, any material
they strike.
When X-rays barely graze a surface, however, they can be reflected from it
in a way that yields an image, although the mirror design is fairly
complex.
For gamma rays no such method of producing an image has yet been
devised. Present-day gamma-ray telescopes simply point in a specified
direction and count photons received.
Telescopes
The arrangement of mirrors in an X-ray telescope allows X-rays
to be reflected at grazing angles and focused to form an
image.