LightLight

Almost all astronomical information is obtained through the light we receive from cosmic objects

GoalsGoals

1)To investigate the nature of light

2)To become familiar with the electromagneticspectrum

3)To introduce telescopes

4)To understand how we collect and study light using telescope

5) Assigned reading: Chapter 6

What is light?What is light?

Light is the part of electromagnetic radiation Light is the part of electromagnetic radiation that humans (and other animals) see that humans (and other animals) see

Light really is a small portion of the Light really is a small portion of the spectrum of electromagnetic radiation spectrum of electromagnetic radiation

Types of electromagnetic radiation differ Types of electromagnetic radiation differ from each other by wavelengthsfrom each other by wavelengths• Blue light: short wavelength; red: long oneBlue light: short wavelength; red: long one• X-ray: very short wavelength; radio: very long X-ray: very short wavelength; radio: very long

oneone Identical situation with sound pitchIdentical situation with sound pitch

• High pitch: short wavelength; bass: long oneHigh pitch: short wavelength; bass: long one

What is Electromagnetic What is Electromagnetic Radiation?Radiation?

Made of propagating waves of electric and Made of propagating waves of electric and magnetic fieldmagnetic field

It carries energy with itIt carries energy with it• Sometimes called “radiant energy”Sometimes called “radiant energy”• Think – solar power, photosynthesis, Think – solar power, photosynthesis,

photo-electric cells, the fireplace …photo-electric cells, the fireplace …

It also carries information It also carries information • the signal received by your car radiothe signal received by your car radio• the signals received by telescopes staring at starsthe signals received by telescopes staring at stars• the signals received by your eyes right now!the signals received by your eyes right now!

What is the electromagnetic wave?What is the electromagnetic wave?

It is electricity and magnetism moving through space.

Light as a waveLight as a wave

Waves you can see:Waves you can see: e.g., ocean waves e.g., ocean waves

Waves you cannot Waves you cannot see:see:• sound wavesound wave• electromagnetic electromagnetic

waves waves

Light is an electromagnetic wave

Properties of WavesProperties of Waves

WavelengthWavelength – the – the distance between distance between crests (or troughs) crests (or troughs) of a wave.of a wave.

FrequencyFrequency – the – the number of crests (or number of crests (or troughs) that pass troughs) that pass by each second.by each second.

SpeedSpeed – the rate at – the rate at which a crest (or which a crest (or trough) moves.trough) moves.

For light in general: λ = c

wavelength

frequency

speed of light = 3x105 km/s in vacuum

Light as particlesLight as particles

• Light comes in quanta of energy Light comes in quanta of energy called called photons photons – little bullets of – little bullets of energy.energy.

• Photons are massless, but they Photons are massless, but they have momentum and they react to have momentum and they react to a gravitational field.a gravitational field.

Wave-particle dualityWave-particle dualityAll types of electromagnetic radiation act as both waves and particles.

The two views are connected by the relation

E=h = h c /

h is the Planck's constantc is the speed of light is the frequency

is the wavelength

IntensityIntensity

A photon's energy depends on the wavelength (or frequency) only, not the intensity.But the energy you experience depends also on the intensity (total number of photons).

It turns out that particles of matter, such as electrons, also behave as both wave and particle.

The theory that describes these puzzles and their solution, and how light and atoms interact is quantum mechanics.

Properties of LightProperties of Light

AllAll light travels through (vacuum) space light travels through (vacuum) space with a velocity = 3x10with a velocity = 3x1055 km/s km/s

The frequency (or wavelength) of photon The frequency (or wavelength) of photon determines how much energy the photon determines how much energy the photon has (E=hhas (E=h).).

The number of photons (how many) The number of photons (how many) determines the intensitydetermines the intensity

Light can be described in terms of either Light can be described in terms of either energy, frequency, or wavelength.energy, frequency, or wavelength.

Visible LightVisible Light ShorterWavelength

LongerWavelength

But visible light isn’t the whole story. It’s just a But visible light isn’t the whole story. It’s just a small part of the entire electromagnetic spectrumsmall part of the entire electromagnetic spectrum

Long Wavelength

(high frequency)(high energy)

Short Wavelength

(low frequency)(low energy)

Electromagnetic RadiationElectromagnetic Radiation

Short wavelength

Long wavelength

Sun seen in optical and Sun seen in optical and UltravioletUltraviolet

Optical Ultraviolet

Sun seen in X-raySun seen in X-ray

X-ray

Matter interacts with light in four Matter interacts with light in four different ways:different ways:

AbsorptionAbsorption – the energy in the photon is absorbed – the energy in the photon is absorbed by the matter and turned into thermal energy by the matter and turned into thermal energy

E.g., Your hand feels warm in front of a fire.E.g., Your hand feels warm in front of a fire.

ReflectionReflection – no energy is transferred and the – no energy is transferred and the photon “bounces” off in a new (and predictable) photon “bounces” off in a new (and predictable) directiondirection

E.g., Your bathroom mirrorE.g., Your bathroom mirror

TransmissionTransmission – no energy is transferred and the – no energy is transferred and the photon passes through the matter unchanged.photon passes through the matter unchanged.

EmissionEmission – matter gives off light in two different – matter gives off light in two different ways. We’ll come back to this next lecture.ways. We’ll come back to this next lecture.

Our eyes work via the process of:Our eyes work via the process of:

transmissiontransmission reflectionreflection absorptionabsorption emissionemission none of the abovenone of the above

A red ball is red because:A red ball is red because:it only emits frequencies it only emits frequencies

corresponding to redcorresponding to red

it only reflects frequencies it only reflects frequencies corresponding to redcorresponding to red

it only transmits frequencies it only transmits frequencies corresponding to redcorresponding to red

it only absorbs frequencies it only absorbs frequencies corresponding to redcorresponding to red

TelescopesTelescopes

The largest optical telescopes in the world:The twin 10-m Keck telescopes (Hawaii)

The HubbleSpace Telescope

The Five College The Five College Radio Astronomy Radio Astronomy

ObservatoryObservatory

The 50-m Large Millimeter TelescopeThe largest radio-telescope in the worldU Mass and Mexico

What telescopes are for?What telescopes are for?Why do they need to be big?Why do they need to be big?

The main feature of a telescope is its capacity The main feature of a telescope is its capacity to collect as to collect as much light as possiblemuch light as possible• Like an antenna: the stronger the signal the clearest the Like an antenna: the stronger the signal the clearest the

transmission.transmission.• Well, guess what: an antenna *is* a telescope (a radio telescope, Well, guess what: an antenna *is* a telescope (a radio telescope,

that is)that is) The larger the light collector, I.e. the primary mirror or lens, The larger the light collector, I.e. the primary mirror or lens,

the more powerful the telescopethe more powerful the telescope• LGP ~ 4 LGP ~ 4 D D22

• LGPLGPAA/LGP/LGPBB = (D = (DAA/D/DBB))22

• A telescope twice as large collects four times as much lightA telescope twice as large collects four times as much light The other primary feature is image The other primary feature is image sharpnesssharpness, to faitfully , to faitfully

reproduce detailsreproduce details• Resolving power: Resolving power: = 11.6/D = 11.6/D

The last, and least important, feature is The last, and least important, feature is magnificationmagnification

Deep Imaging of the sky:Deep Imaging of the sky:at the edge of the Universeat the edge of the Universe

Subaru + SUPREME HST + ACS

To study galaxy formation both space-based sensitivity and angular resolution required!!Note how many more details and faint objects can be observed with the Hubble Space Telescope

Different types of telescopesDifferent types of telescopes

To detect different types (wavelengths) of light, To detect different types (wavelengths) of light, eg. X-ray, UV, optical, infrared, radio, different eg. X-ray, UV, optical, infrared, radio, different technologies are requiredtechnologies are required

For example, special mirrors are necessary for For example, special mirrors are necessary for X-ray telescopes or else the radiation would X-ray telescopes or else the radiation would pass through them.pass through them.

Hence, it is necessary to specialize telescopes Hence, it is necessary to specialize telescopes to the wavelength of light one wishes to study.to the wavelength of light one wishes to study.

We X-ray, UV, optical, infrarerd and radio We X-ray, UV, optical, infrarerd and radio telescopes telescopes

Different locations for telescopesDifferent locations for telescopes In addition, the Earth’s atmosphere affects light of different In addition, the Earth’s atmosphere affects light of different

wavelengths differently:wavelengths differently:1.1. It totally absorbs X-ray and UV light: X-ray and UV telescopes MUST It totally absorbs X-ray and UV light: X-ray and UV telescopes MUST

be placed in spacebe placed in space

2.2. It blurs the optical light, I.e. it destroys sharpness. It blurs the optical light, I.e. it destroys sharpness.

3.3. It also adds the glare of the night sky (yup! There is such thing) to It also adds the glare of the night sky (yup! There is such thing) to optical and infrared light, which makes faint sources hard to see.optical and infrared light, which makes faint sources hard to see.

4.4. It totally absorbs some (important) infrared lightIt totally absorbs some (important) infrared light• As a consequence some telescopes can operate on the ground:As a consequence some telescopes can operate on the ground:

• optical, near-infrared, radiooptical, near-infrared, radio• Some can only work in spaceSome can only work in space

• X-ray, UV, mid- and far-infraredX-ray, UV, mid- and far-infrared• For high-resolution (super-sharp) observations, or for For high-resolution (super-sharp) observations, or for

observations of very faint sources (i.e. to avoid the glare of the observations of very faint sources (i.e. to avoid the glare of the Earth’s atmospherer) either space telescopes or very advanced Earth’s atmospherer) either space telescopes or very advanced technologies (adaptive optics) are required.technologies (adaptive optics) are required.

In fact, most wavelengths cannot penetrate In fact, most wavelengths cannot penetrate the Earth's atmospherethe Earth's atmosphere

Why different wavelengths are Why different wavelengths are requiredrequired

Regardless of the technology, different Regardless of the technology, different wavelengths carries different information:wavelengths carries different information:• Shorter wavelengths carry information on Shorter wavelengths carry information on

very energetic phenomena (e.g. black holes, very energetic phenomena (e.g. black holes, star formation)star formation)

• Optical wavelengths carry information on the Optical wavelengths carry information on the structures of galaxies and their motions (the structures of galaxies and their motions (the assembly of the bodies of galaxies, their size)assembly of the bodies of galaxies, their size)

• Longer wavelengths carry information on the Longer wavelengths carry information on the chemical composition, physical state (gas chemical composition, physical state (gas and dust, presence, chemical elements; and dust, presence, chemical elements; temperature)temperature)

Wavelengths and size of thingsWavelengths and size of things

Optical SkyOptical Sky

Radio SkyRadio Sky

Soft X-ray SkySoft X-ray Sky

Telescope InstrumentsTelescope Instruments

Cameras:Cameras:• To obtain images at desired wavelength or To obtain images at desired wavelength or

wavelengths (color images)wavelengths (color images)• This yields the morphology, size of the sourcesThis yields the morphology, size of the sources

Spectrographs:Spectrographs:• To study the intensity of the various To study the intensity of the various

wavelengths (colors)wavelengths (colors)• This yields the physical nature (star, galaxy, This yields the physical nature (star, galaxy,

balck hole), chemical composition, physical balck hole), chemical composition, physical properties (temperature, density), dynamics properties (temperature, density), dynamics (motions, mass), distance of the sources (motions, mass), distance of the sources

Intensity(spatial distribution of the light) Spectra

(composition of the objectand the object’s velocity)

There are three basic aspects ofthe light from an object that we can study from the Earth.

Variability(change with time)

Spectral Lines of Some ElementsSpectral Lines of Some Elements

 Argon 

                                                                                

 Helium

                                                                                 

 Mercury

                                                                                 

 Sodium

                                                                                 

 Neon

                                                                                 

Spectral lines are like a cosmic barcode system for elements.

Life at the telescope. ILife at the telescope. I

The trusty Night Assistant, who does all the work

The telescope, before sunsetThe MMT 6.5-m telescope, Univ. of Arizona

Life at the telescope. IILife at the telescope. II

The diligent Student,who makes sure the work is done right

The hard-working Professor, who bosses everybody around