Light: The Cosmic Messenger

What is light?

Does the prism PRODUCE colors, or were they there already in the light? How can you tell?

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

Galileo tried to measure with lanterns! …but light was too quick! 1 mile there & back in seconds! Observation: Light is FAST!

Roemer tried to measure it with Jupiter’s Moons! …by timing when they passed in front or behind Jupiter Observation: Light is FAST!

Hubble Space Telescope image of Ganymede being eclipsed by Jupiter Observation: Light is FAST!

More Observations: Speed of Light?

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

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

Observation: Water Waves naturally interfere & create noticeable patterns

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

Wavelength and Frequency wavelength  frequency = speed of a wave

Wavelength of light? Microwaves on buttered bread!?? See: The Naked Scientists PodcastThe Naked Scientists Podcast

Observations of Nature  Electricity acts through space over a distance  Lightening, sparks on a doorknob

Observations of Nature 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  Portable gas generators

More Observations  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!

Electro-magnetic radiation! The Wave Model of Light

Light is an electromagnetic wave.

The Electromagnetic Spectrum

Still More Observations of Nature  Einstein’s Photo-electric Effect showed light can eject individual electrons  Energy depended upon light’s color! Metal plate Single electrons emerge Light shining onto metal

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

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

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

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

Atmospheric “Windows” to the stars & universe: Visible & Radio light

Different types of Reflecting Telescopes

Small Telescope image of Andromeda Galaxy

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

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

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

Orion in UV, Infrared, & Optical Wavelengths

#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!

#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

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

“Seeing” is the ability to resolve 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!

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

1.Ground-based image of Neptune 2.Ground-based image with adaptive optics 3.Hubble Space Telescope image 123

#3 Function: Magnification  Least important  Without a bright, sharp image, no use!  Bigger, Dimmer, Fuzzier!  Depends upon EYEPIECE used  Small scopes: $ each  Easily swapped to magnify images  Depends upon telescope geometry, too

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 Tracking motions of stars at Milky Way Center

SALT in Africa  Largest current “single” surface scope Largest current “single” surface scope

Next Generation Space Telescope  NASA’s next great observatory  Bigger than Hubble Bigger than Hubble

Seeing in Stereo!

Interferometry – 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…

What is matter?

Atomic Terminology  Atomic Number = # of protons in nucleus  Atomic Mass Number = # of protons + neutrons

Atomic Terminology  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 of Light with Matter Interactions between light and matter determine the appearance of everything around us.

How do light and matter interact?  Emission  Absorption  Transmission: — 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 - 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?

Learning from Light  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)

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.  Each transition corresponds to a unique photon energy, frequency, and wavelength. Energy levels of hydrogen

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? ABCDE

ABCDE

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

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

Thought Question Which letter(s) labels emission lines? ABCDE

ABCDE

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, X-ray, & 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! 1.Gather Light 2.Resolve Sharp Details 3.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

Small Telescope image of Andromeda Galaxy

Larger Telescope 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.Ground-based image of Neptune 2.Ground-based image with adaptive optics 3.Hubble Space Telescope image 123

#3 Function: Magnification  Least important  Without a bright, sharp image, no use!  Bigger, Dimmer, Fuzzier!  Depends upon EYEPIECE used  Small scopes: $ 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 & 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 Tracking motions of stars at Milky Way Center

SALT in Africa  Largest current “single” surface scope Largest current “single” surface scope

Next Generation Space Telescope  NASA’s next great observatory  Bigger than Hubble Bigger than Hubble

Seeing in Stereo!

Interferometry – 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 Telescope.

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.

Key Terms 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 DID YOU THINK?  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 DID YOU THINK?  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 DID YOU THINK?  What is the main purpose of a telescope?  A telescope is designed primarily to collect as much light as possible.

WHAT DID YOU THINK?  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.

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