




What is “visible” light?
 A wave? A particle? Energy?
What is electromagnetic radiation?
What are the main functions of telescopes?
Why do all research telescopes use mirrors, rather than lenses, to collect light?
How can we know anything about stars’ temperatures, sizes, ages, & lives?

 Connection between visible light, X-rays, radio waves, & other electromagnetic radiation
 Debate over what light is & how Einstein resolved it
 How telescopes collect and focus light
 Why different types of telescopes are used for different types of research

 Limitations of telescopes, especially those that use lenses to collect light
 New generations of land-based and space-based high-technology telescopes
 How astronomers use entire spectrum of electromagnetic radiation to observe stars & other astronomical objects and events

Observation: Color is property of light!
White light is composed of different colors when shone through glass….
…but the glass is not creating those colors!

More Observations: Speed of Light?

More Observations: Speed of Light is not infinite!
Roemer’s observations of Jupiter’s Moon’s Eclipses demonstrated light moves at a finite speed

Even More Observations: Light has
“energy”
Non-visible light
(beyond the red end of the spectrum) has energy, too!

Observation: Water Waves naturally interfere & create noticeable patterns

Observation: Light has a wavelike property, too!
Young’s Experiment (1801)

wavelength
 frequency = speed of a wave

 Electricity acts through space over a distance
 Lightening, sparks on a doorknob
 Magnetism acts through space over a distance
 Two magnets attract or repel one another without touching

 If you spin a conductor in a magnetic field, you get electricity!
 Electric Generators @ dams & windmills
 Portable gas generators
 If you run electricity into a coil, you get a magnet!

“Electromagnetic” cranes
 Auto solenoids
 Electric Motors

 Change Electricity => create magnetism
 Change Magnetism => create electricity
 Continuously change both, continuously create radiation!

Radiation created moves at “c” – the speed of light!

Electro-magnetic radiation!

Light is an electromagnetic wave.

The Electromagnetic Spectrum


Einstein’s Photo-electric Effect showed light can eject individual electrons

Energy depended upon light’s color!
Light shining onto metal
Single electrons emerge
Metal plate


Planck’s Energy Curve showed light can be modeled with specific “quanta” of energy

Peak depended upon light’s color!
Metal plate

 Particles of light are called photons .
 Each photon has a wavelength and a frequency.
 The energy of a photon depends on its frequency.

l  f = c l
= wavelength, f = frequency c = 3.00

10 8 m/s = speed of light
E = h
 f = photon energy h = 6.626

10 −34 joule
 s
Planck’s constant

•
•
•
The higher the photon energy, the longer its wavelength.
the shorter its wavelength.
Energy is independent of wavelength.

•
•
•
The higher the photon energy , the longer its wavelength.
the shorter its wavelength.
Energy is independent of wavelength.
•
• X-rays can kill you!
Country-western music on the radio can’t

The entire EM
Spectrum
What we “see” is only a small part of what there is!

EM Spectrum
Varies by…
Size (wavelength, color)
Energy
How the waves are detected
But not….
How fast they move through space!

 Atomic Number = # of protons in nucleus
 Atomic Mass Number = # of protons + neutrons

 Isotope: same # of protons but different # of neutrons ( 4 He, 3 He)
• Molecules : consist of two or more atoms (H
2
O, CO
2
)

Interactions between light and matter determine the appearance of everything around us.

 Emission
 Absorption
 Transmission:
—
—
Transparent objects transmit light.
Opaque objects block (absorb) light.
 Reflection or scattering

Mirror reflects light in a particular direction.
Movie screen scatters light in all directions.

•
•
•
•
Why is a rose red ?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

•
•
•
•
Why is a rose red?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

 What are the three basic types of spectra ?
 How does light tell us composition - what things are made of?
 How does light tell us the temperatures of planets and stars?
 How does light tell us the speed of a distant object towards or away from us?

 Spread light out with prism or grating:
 Hot solids give off rainbows
 Hot gases give off bright lines of particular color
 Cool gases in front of a hot solid show dark shadows over particular colors (only)

 The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.

 A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

 A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of these three basic types.

Spectrum of the Sun

 Each type of atom has a unique set of energy levels.
Energy levels of hydrogen
 Each transition corresponds to a unique photon energy, frequency, and wavelength.

 Downward transitions produce a unique pattern of emission lines.

 Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.

 Each type of atom has a unique spectral fingerprint.

 Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

Thought Question
Which letter(s) labels absorption lines?
A B C D E

Thought Question
Which letter(s) labels absorption lines?
A B C D E

Thought Question
Which letter(s) labels the peak (greatest intensity) of infrared light?
A B C D E

Thought Question
Which letter(s) labels the peak (greatest intensity) of infrared light?
A B C D E

Thought Question
Which letter(s) labels emission lines?
A B C D E

Thought Question
Which letter(s) labels emission lines?
A B C D E

 Where must telescopes be placed to observe the universe in different wavelengths?
 What are the basic types of telescopes?
 What 3 functions do ALL telescopes do?
 How can we combine observations to get even more detail?

Gather Radio waves & Visible Light on the Ground

Infra-red, UV, Xray, & Gamma Rays can’t reach the ground

Atmospheric “Windows” to the stars & universe: Visible & Radio light
Optical & Radio Telescope observatories on Earth
Other wavelength telescopes launched ABOVE atmosphere

Refracting Telescopes bend light through lenses
Heavy glass lenses, bending different colors to different points (“ Chromatic aberration ”) & imperfections in glass, limit practical size

Reflecting Telescopes bounce light off mirrors

Different types of Reflecting Telescopes

1.
2.
3.
Gather Light
Resolve Sharp Details
Magnify Resulting Images
Regardless of Wavelength range & size

 Depends upon the size of the objective mirror or lens.
 Light gathering area increases with SQUARE of the diameter
 10 m telescope gather 4x more light than 5m
 Subject to interference from other sources!

Tucson, 1959
Tucson, 1989

Small
image of
Andromeda Galaxy

Larger
image of
Andromeda Galaxy

 Depends upon the size of the objective mirror or lens.
 Better resolution with more light
 Depends upon wavelength of light, too!
 Smaller wavelengths provide smaller details
 UV images have more detail than Radio
 Also subject to interference

Resolution is the ability to see small details
Affected by:
• Imperfections in optics (shapes of lenses/mirrors)
• Atmospheric motion, density, temperature, moisture
Improved by:
Adaptive optics “subtracting out” the atmospheric effects
Getting above atmosphere!

Radio Telescopes gather long-wave, low-energy light
Poor resolution unless made LARGE!

Improve resolution by getting above the atmosphere
(and gather more types of light, too!)

1 2 3
1. Ground-based image of Neptune
2. Ground-based image with adaptive optics
3. Hubble Space Telescope image

 Least important
 Without a bright, sharp image, no use!
 Bigger, Dimmer, Fuzzier!
 Depends upon EYEPIECE used
 Small scopes: $50-500 each
 Easily swapped to magnify images
 Depends upon telescope geometry, too

Magnify this…
To THIS

Photographs vs. CCD chips vs.
Multi-color filtered CCD composite images

Orion in UV, Infrared, & Optical Wavelengths


Active optics (1980’s)

Put actuators on segmented mirrors to “bend” them to the right shape
 Keck, NTT, VLT Telescopes

Adaptive optics (1990’s to present)

“Deform” mirror in real time to compensate for atmospheric motion
 Laser Guide Stars

 (4) combined 8.2 m telescopes
 Tracking motions of stars at Milky Way Center


Largest current “single” surface scope


NASA’s next great observatory
 Bigger than Hubble

Interferometry – Combining signals simultaneously from 2 or more scopes

Visible & Radio wave views of Saturn

We already have enough!
Why do we need a more detailed picture of Mars?
Who cares?
This cost $100 Million dollars? You’ve got to be kidding me…

 Photons, units of vibrating electric and magnetic fields, all carry energy through space at the same speed, the speed of light (300,000 km/s in a vacuum, slower in any medium).
 Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays are the forms of electromagnetic radiation. They travel as photons, sometimes behaving as particles, sometimes as waves.

 Visible light occupies only a small portion of the electromagnetic spectrum.
 The wavelength of a visible light photon is associated with its color. Wavelengths of visible light range from about 400 nm for violet light to 700 nm for red light.
 Infrared radiation and radio waves have wavelengths longer than those of visible light. Ultraviolet radiation, X rays, and gamma rays have wavelengths that are shorter.


A telescope’s most important function is to gather as much light as possible. Its second function is to reveal the observed object in as much detail as possible. Often the least important function of a telescope is to magnify objects.
 Reflecting telescopes, or reflectors, produce images by reflecting light rays from concave mirrors to a focal point or focal plane.

 Refracting telescopes, or refractors, produce images by bending light rays as they pass through glass lenses.
Glass impurity, opacity to certain wavelengths, and structural difficulties make it inadvisable to build extremely large refractors.
 Reflectors are not subject to the problems that limit the usefulness of refractors.
 Earth-based telescopes are being built with active and adaptive optics. These advanced technologies yield resolving power comparable to the Hubble Space
Telescope.

 Radio telescopes have large, reflecting antennas
(dishes) that are used to focus radio waves.
 Very sharp radio images are produced with arrays of radio telescopes linked together in a technique called interferometry.

Earth’s atmosphere is fairly transparent to most visible light and radio waves, along with some infrared and ultraviolet radiation arriving from space, but it absorbs much of the electromagnetic radiation at other wavelengths.

 For observations at other wavelengths, astronomers mostly depend upon telescopes carried above the atmosphere by rockets. Satellite-based observatories are giving us a wealth of new information about the universe and permitting coordinated observation of the sky at all wavelengths.
 Charge-coupled devices (CCDs) record images on many telescopes used between infrared and X-ray wavelengths.

active optics adaptive optics angular resolution
Cassegrain focus charge-coupled device coudé focus electromagnetic radiation electromagnetic spectrum eyepiece lens focal length focal plane focal point frequency gamma ray
infrared radiation interferometry light-gathering power magnification
Newtonian reflector objective lens photon pixel primary mirror prime focus radio telescope radio wave reflecting telescope reflection refracting telescope
Schmidt corrector plate secondary mirror seeing disk spectrum spherical aberration twinkling ultraviolet (UV) radiation very-long-baseline interferometry (VLBI) wavelength
X ray

 What is light?
 Light —more properly “visible light,” is one form of electromagnetic radiation. All electromagnetic radiation
(radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays) has both wave and particle properties.

 What type of electromagnetic radiation is most dangerous to life?
 Gamma rays have the highest energies of all photons, so they are the most dangerous to life. However, ultraviolet radiation from the Sun is the most common everyday form of dangerous electromagnetic radiation that we encounter.

 What is the main purpose of a telescope?
 A telescope is designed primarily to collect as much light as possible.

 Why do all research telescopes use mirrors, rather than lenses, to collect light?
 Telescopes that use lenses have more problems, such as chromatic aberration, internal defects, complex shapes, and distortion from sagging, than do telescopes that use mirrors.

 Why do stars twinkle?

Rapid changes in the density of Earth’s atmosphere cause passing starlight to change direction, making stars appear to twinkle.