Electromagnetic Radiation and Optical Telescopes

1
Electromagnetic Radiation and Optical Telescopes
2
Properties of Electrons, Protons, and
Neutrons(The Main Constituents of Ordinary
Matter)

• MassElectrons have a mass of 9.1110-31kg.The
  mass of a proton is 1836 times the mass of an
  electron.The mass of the neutron is 1839 times
  the mass of the electron.
• ChargeElectrons are negatively charged.Protons
  are positively charged.Neutrons are uncharged.

3
How do electrically-charged particles influence
each other?

• There are two kinds of electrical charge
  positive and negative.
• Charges with the same sign repel each other.
• Charges with opposite signs attract each other.
• Uncharged particles exert no electrical force on
  each other or on charged particles.
• The arrows in the diagram show the direction of
  the force. The equation, which is called
  Coulombs Law, is the mathematical description
  of the strength of the electrostatic interaction
  between charged particles.

The fact that the Coulomb force between two
charged particles is directly proportional to the
product of the charges and inversely proportional
to the square of the distance between them is
useful for understanding nuclear fusion reactions
in stars.
kc is the Coulomb constant. q1 and q2 are the
charges. r is the distance between them.
4
What is an electric field?
An electric field is the property of space by
means of which one electrically - charged
particle exerts a force on another
electrically-charged particle.
The charge of particle 1 changes the space around
it, giving it the property we call the electric
field. The electric field of particle 1 exerts a
force on particle 2.
The electric force on a positively charged
particle is in the direction of the electric
field.
What is a magnetic field?
A magnetic field is the property of space by
means of which one moving charged particle exerts
a magnetic force on another moving charged
particle.
The motion of particle 1 produces a magnetic
field in space. That magnetic field exerts a
magnetic force on moving particle 2.
The magnetic force on a moving charged particle
is in the direction perpendicular to the magnetic
field and to the particles velocity. Because of
this, charged particles tend to spiral around
magnetic field lines.
5
(No Transcript)
6
Waves
A wave is a process in which energy and momentum
travel from one point to another without the
motion of mass between the two points. A wave
transfers a disturbance rather than matter.

• Water wavesA pebble dropped into some water
  disturbs the water, causing ripples to move away
  from the point of impact. The disturbance is an
  up and down motion of the water.
• Sound WavesThe vibration of the head of a drum
  alternately compresses and rarifies the air in
  contact with it. The series of compressions and
  rarefactions moves away from the drumhead. When
  these compressions and rarefactions reach an
  eardrum, the eardrum moves back and forth with
  the same frequency as the drumhead.

7
Electromagnetic Waves
An electromagnetic wave consists of oscillating
electric and magnetic fields.
As an electromagnetic wave passes a charged
particle, the electric field at the particle
varies with time, dragging the particle back and
forth along the direction of the field.
Direction of wave motion
Light is electromagnetic radiation that is
visible to the human eye. However, the terms
light and electromagnetic radiation are often
used interchangeably.
8
Wavefronts and Rays
In an electromagnetic wave, surfaces of constant
electric field are called wave fronts. When the
wave comes from a very large distance (from a
star, for example), the wave fronts are planes as
shown in the following figure.
The long yellow arrow shows the direction in
which the wave travels. It is perpendicular to
the wave fronts. Lines parallel to the arrow are
called rays.
The short red arrows show the direction of the
electric field. This direction alternates from
one plane to another and is zero half way between
planes.
When wavefront 1 reaches a proton, it accelerates
the proton downward, when wavefront 2 reaches a
proton, it accelerates the proton upward, etc.
The directions are reversed for electrons.
9
Properties of Electromagnetic Waves

• Wavelength (l) is the distance between two
  successive peaks or two successive valleys of the
  wave.
• Frequency (f) is the number of oscillations per
  second, or the number of wavefronts with the
  maximum electric field that pass a given point in
  one second. One oscillation per second is called
  a Hertz (Hz).
• The speed (c) of an electromagnetic wave in
  vacuum is 3.00108 meters per second.
• f c / l is the relationship between frequency
  and wavelength for an electromagnetic wave.
• Photons are particles of light i.e., discrete
  bundles of electromagnetic energy.

10
The Electromagnetic Spectrum
Electromagnetic radiation with wavelengths
between 400 nm and 700 nm is visible. In order of
decreasing wavelength (increasing frequency), the
colors are red (700 nm), orange, yellow, green,
blue, violet (400 nm).
11
Atmospheric Windows

• Most kinds of electromagnetic radiation are
  absorbed or reflected in Earths upper atmosphere
  and dont reach telescopes on the ground.
• For example, carbon dioxide and water vapor
  absorb most infrared radiation and the ozone
  layer absorbs most ultraviolet.
• Visible electromagnetic radiation (light), some
  short wavelength infrared, and some radio waves
  penetrate the atmosphere.
• The wavelength ranges for which Earths
  atmosphere is relatively transparent are called
  atmospheric windows.

12
Types of Electromagnetic Radiation

• Visible (400 nm 700 nm)
• Near infrared (1200 nm 40000 nm)
• Far infrared (more than 40000 nm)
• Near Ultraviolet (400 nm 290 nm)
• Far ultraviolet (290 nm 10 nm)
• X rays ( 10 nm 10 0.01 nm)
• Gamma rays (less than 0.01 nm)

13
Brightness and Intensity
How bright you perceive a source of light to be
depends on the rate at which light energy enters
your eye. The rate of transfer of energy is
specified in terms of a unit called the watt. A
100-watt light bulb, for example, is twice as
bright as a 50-watt light bulb.
The rate at which light energy enters your eye
depends on the energy transferred by the
electromagnetic wave across a unit of area in a
unit of time.
The rate at which an electromagnetic wave
transfers energy across a unit of area in a unit
of time is called the intensity of the wave.
14
Refraction is the bending of a ray of light when
it passes from one material to another
The solid red lines represent light rays from the
bottom end of the brown stick.
Stick
Air
Water
When light crosses a boundary into a region where
its speed is higher, its path is bent away from
the line normal (perpendicular) to the boundary.
When light crosses a boundary into a region where
its speed is lower, its path is bent toward the
line normal (perpendicular) to the boundary.
15
Image Formation by a Biconvex Lens
Object
Focal point
The law of refraction implies the following rules
for image formation in a lens with two convex
surfaces.
A ray parallel to the optic axis must pass
through the focal point.
A ray that passes through the center of the lens
continues in the same direction.
The intersection of these two rays determines the
image of the point from which the rays originated.
16
Image Formation by a Spherical Concave Mirror
C
F
Optic Axis
1

A ray of light that is parallel to the optic axis
is reflected through the focal point.
A ray of light that passes through the center of
curvature is reflected back through the center of
curvature.
The intersection of the two rays locates the
image.
17
The Three Powers of a Telescope

• Light-gathering power (area of the objective).
• Magnifying power (magnification).
• Resolving power (resolution).

18
Light - Gathering Power
LGPA light- gathering power of A DA diameter
of A LGPB light- gathering power of B DB
diameter of B
ExampleCompare the light-gathering power of the
Hale telescope to that of our 14-inch ( 35.56 cm)
LX200 telescopes.
19
Resolving Power
20
Resolution
The resolution or resolving angle of a telescope
is the angular size of the smallest detail that
can be detected in the image formed by the
telescope.
It is directly proportional to the wavelength
observed and inversely proportional to the
aperture of the telescope.
a is in arcseconds, l and D are in the same
distance units.
ExampleThe aperture of our Meade LX200
telescopes is 35.6 cm. What is the angle of
resolution for green light (l 550 nm).
D 35.6 cm
l 550 nm
1 nm 1 nanometer 10-9 m
l 55010-9 m
l 55010-9100 cm
l 5.510-5 cm
NoteUnless adaptive optics are used, atmospheric
turbulence limits resolution to 0.5 arcsecond or
more.
a 0.32"
21
Characteristics of the Ideal Telescope

• High light-gathering power.
• Small angle of resolution.
• Accurate tracking and pointing.

The first two require a large aperture. The third
requires good right ascension and declination
motors as well as a rigid mount.
22
Reasons for Preferring Reflectors

• In a refractor, light must pass through the lens,
  so the glass must be free of bubbles and other
  flaws. For large apertures, this is difficult
  and expensive.
• For the same reason, dispersion (dependence of
  the focal point on wavelength) smears the image
  in a refractor, but not in a reflector.
• Lenses can only be supported around their edges,
  while mirrors can be supported from behind.

23
8.1-Meter Northern Gemini Telescope (Mauna Loa)
24
10-Meter Keck Mirrors (Mauna Loa)
25
8.4-m Large Binocular Telescope (Mount Graham,
Arizona)
26
6-Meter Liquid Mirror Telescope (The University
of British Columbia)
27
Very Large Telescope Array (Four 8.2-Meter
Telescopes)
28
42-Meter European Extremely-Large Telescope
29
Conditions Necessary for a Good Observatory
Location

• High percentage of clear nights
• High and dry (deserts and mountaintops)Recall
  that water vapor absorbs IR).Even dry air
  absorbs some em radiation
• At least a hundred miles from any large city to
  avoid light pollution
• Good seeing i.e., low smearing of telescope
  images by atmospheric turbulence.

30
Light Pollution in the USA
31
Mauna Kea Observatory
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
Infrared, Radio, X-Ray, and Gamma Ray Telescopes
37
Arecibo Radio Telescope
http//www.naic.edu/
38
Very Large Array
http//www.vla.nrao.edu/
39
Wide View of the VLA
40
Very Long Baseline Array
http//www.vlba.nrao.edu/
41
100-Meter Green Bank Radio Telescope
http//www.gb.nrao.edu/
42
Sagittarius A (Center of the Milky Way Galaxy)
43
Chandra X Ray Observatory
http//chandra.harvard.edu/
44
Centaurus A (Active Galaxy with a Supermassive
Black Hole at its Center(about 11 million ly
from Earth)
45
Compton Gamma Ray Observatory
http//cossc.gsfc.nasa.gov/docs/cgro/index.html
46
Gamma Ray Burst
47
Spitzer Infrared Space Telescope
http//www.spitzer.caltech.edu/
48
W5 star-forming region(6500 ly from Earth)
49
Swift Multi-Wavelength Orbiting
Observatory(Gamma Ray, X Ray, Ultraviolet/Optical
)
50
X-ray Outburst of Soft Gamma-ray Repeater 1E
1547.0-5408