Light: The Cosmic Messenger


Light: The Cosmic


What is light?


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?

In this chapter you will discover…

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

In this chapter you will discover…

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


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 and Frequency


 frequency = speed of a wave

Observations of Nature

Electricity distance acts through space over a

Lightening, sparks on a doorknob

Magnetism acts through space over a distance

Two magnets attract or repel one another without touching

More Observations

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

Maxwell’s Observations

Change Electricity => create magnetism

Change Magnetism => create electricity

Continuously change both, continuously create radiation!

Radiation created moves at “c” – the speed of light!

The Wave Model of Light

Electro-magnetic radiation!

Light is an

electromagnetic wave.

The Electromagnetic Spectrum

Still More Observations of Nature


Photo-electric Effect

showed light can eject

individual electrons

Energy depended upon light’s color!

Light shining onto metal

Single electrons emerge

Metal plate

Still More Observations of Nature

Planck’s Energy Curve showed light can be modeled with specific “quanta” of energy

Peak depended upon light’s color!

Metal plate

The Particle Model of Light

Particles of light are called

photons .

Each photon has a wavelength and a frequency.

The energy of a photon depends on its frequency.

Wavelength, Frequency, and



l 


= l

= wavelength,

f c

= frequency

= 3.00


8 m/s = speed of light

E = h


= photon energy


= 6.626


−34 joule

Planck’s constant s

Thought Question

The higher the photon energy, the longer its wavelength.

the shorter its wavelength.

Energy is independent of wavelength.

Thought Question

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


What we “see” is only a small part of what there is!

EM Spectrum

Varies by…

Size (wavelength, color)


How the waves are detected

But not….

How fast they move through space!

What is matter?

Atomic Terminology

Atomic Number = # of protons in nucleus

Atomic Mass Number neutrons

= # of protons +

Atomic Terminology

Isotope: same # of protons but different # of neutrons (





• Molecules

: consist of two or more atoms (H





Interactions of Light with


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

How do light and matter interact?




Transparent objects transmit light.

Opaque objects block (absorb) light.

Reflection or scattering

Reflection and Scattering

Mirror reflects light in a particular direction.

Movie screen scatters light in all directions.

Thought Question

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.

Thought Question

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.

Learning from Light

What are the three basic types of spectra ?

How does light tell us composition things are made of?

- what

How does light tell us the temperatures planets and stars?


How does light tell us the speed of a distant object towards or away from us?

Learning from Light

Spread light out with prism or grating:

Hot solids give off rainbows

Hot gases color give off bright lines of particular

Cool gases

in front of a hot solid

show dark shadows over particular colors (only)

Continuous Spectrum

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

Emission Line Spectrum

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.

Absorption Line Spectrum

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

Three basic types of spectra

Continuous Spectrum

Emission Line Spectrum

Absorption Line Spectrum

Spectra of astrophysical objects are usually combinations of these three basic types.

How does light tell us what things are made of?

Spectrum of the Sun

Chemical Fingerprints

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.

Chemical Fingerprints

Downward transitions produce a unique pattern of emission lines.

Chemical Fingerprints

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

Chemical Fingerprints

Each type of atom has a unique spectral fingerprint.

Chemical Fingerprints

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

Example: Solar Spectrum

Thought Question

Which letter(s) labels absorption lines?


Thought Question

Which letter(s) labels absorption lines?




Thought Question

Which letter(s) labels the peak (greatest intensity) of infrared light?


Thought Question

Which letter(s) labels the peak (greatest intensity) of infrared light?



Thought Question

Which letter(s) labels emission lines?


Thought Question

Which letter(s) labels emission lines?



Gathering Light

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

Functions of ALL Telescopes!




Gather Light

Resolve Sharp Details

Magnify Resulting Images

Regardless of Wavelength range & size

#1 Function: Gathering Light

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



image of

Andromeda Galaxy



image of

Andromeda Galaxy

#2 Function: Resolution

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

#3 Function: Magnification

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…


Photographs vs. CCD chips vs.

Multi-color filtered CCD composite images

Orion in UV, Infrared, & Optical Wavelengths

Active & Adaptive Optics!

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

VLT in Chile

(4) combined 8.2 m telescopes

Tracking motions of stars at Milky Way Center

SALT in Africa

Largest current “single” surface scope

Next Generation Space Telescope

NASA’s next great observatory

Bigger than Hubble

Seeing in



– Combining signals simultaneously from 2 or more scopes

Visible & Radio wave views of Saturn

Why build telescopes at all?

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…

Summary: The Nature Of Light

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.

The Nature Of Light

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.

Optics and Telescopes

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.

Optics and Telescopes

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


Nonoptical Astronomy

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.

Nonoptical Astronomy

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

Key Terms

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?


—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.