What is Light?What is Light?

Properties of WavesProperties of Waves

All electromagnetic waves travel at the same speedThe speed of light: 300,000,000 m/s

trough

crest

Wavelength

(length/cycle)

Wavelength (): the length of one complete cycle

trough

crest

Properties of WavesProperties of Waves

All electromagnetic waves travel at the same speedThe speed of light: 300,000,000 m/s

Wavelength

(length/cycle)

Amplitude: 1/2 height between trough and crest

Amplitude

trough

crest

Properties of WavesProperties of Waves

All electromagnetic waves travel at the same speedThe speed of light: 300,000,000 m/s

Wave Speed = wavelength x frequency

Wave Speed = f

(length/second) = (length/cycle) x (cycle/second)

Light as a WaveLight as a Wave

Light waves are characterized by a wavelength

and a frequency f

c = 300,000 km/s = 3108 m/s

ll

Electromagnetic WavesElectromagnetic Waves

Light as a WaveLight as a WaveWavelengths of light are measured in units of

nanometers (nm) or Ångström (Å):1 nm = 10-9 m

1 Å = 10-10 m = 0.1 nm

Visible light has wavelengths between 4000 Å and 7000 Å

(400 nm – 700 nm).

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes

or satellites

Wavelengths and ColorsWavelengths and Colors

Different colors of visible light correspond to different wavelengths.

Visible Light (VIS)

700 to 400 nm

Our eyes are sensitive to this region of the spectrum

Red-Orange-Yellow-Green-Blue-Indigo-Violet

Solar Radiation

The Source for 99.9% of Earth’s

Energy

Solar SpectrumSolar Spectrum

The sun emits radiation at all wavelengths

Most of its energy is in the IR-VIS-UV

portions of the spectrum

~50% of the energy is in the visible region

~40% in the near-IR ~10% in the UV

Wavelength (m)

RADIO WAVESRADIO WAVES Have the Have the longestlongest wavelengths and wavelengths and

lowest lowest frequencies of all the electromagnetic waves.frequencies of all the electromagnetic waves. A radio picks up radio waves through A radio picks up radio waves through

an antenna and converts it to an antenna and converts it to sound waves.sound waves.

Each radio station in an area broadcasts Each radio station in an area broadcasts at a different frequency. # on radio at a different frequency. # on radio dial tells frequency.dial tells frequency.

MRI MRI (MAGNETIC RESONACE IMAGING)(MAGNETIC RESONACE IMAGING)

Uses radio waves with a Uses radio waves with a magnet to create an imagemagnet to create an image

MICROWAVESMICROWAVES MicrowavesMicrowaves—have the shortest wavelengths —have the shortest wavelengths

and the highest frequency of the and the highest frequency of the radio waves.radio waves. Used in microwave ovens.Used in microwave ovens.

• Waves transfer energy to the water in the food causing Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the them to vibrate which in turn transfers energy in the form of heat to the food.form of heat to the food.

Used by Used by cell phones and pagers.cell phones and pagers. RADARRADAR ( (RaRadio dio DDetection etection aand nd RRanging)anging)

• Used to find the speed of an object by sending out radio Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.waves and measuring the time it takes them to return.

INFRARED RAYSINFRARED RAYS InfraredInfrared= below red= below red Shorter wavelength and higher Shorter wavelength and higher

frequency than microwaves.frequency than microwaves. You can feel the longest ones You can feel the longest ones

as warmth on your skinas warmth on your skin Heat lamps give off infrared Heat lamps give off infrared

waves.waves. Warm objects give off more Warm objects give off more

heat energy than cool objects.heat energy than cool objects. ThermogramThermogram—a picture that —a picture that

shows regions of different shows regions of different temperatures in the body. temperatures in the body. Temperatures are calculated by Temperatures are calculated by the amount of infrared radiation the amount of infrared radiation given off. Therefore people given off. Therefore people give off infrared rays.give off infrared rays.

VISIBLE LIGHTVISIBLE LIGHT Shorter wavelength and higher frequency than infrared rays.Shorter wavelength and higher frequency than infrared rays. Electromagnetic waves we Electromagnetic waves we can see.can see. Longest wavelength= red lightLongest wavelength= red light Shortest wavelength= violet (purple) lightShortest wavelength= violet (purple) light When light enters a new medium it bends (refracts). Each When light enters a new medium it bends (refracts). Each

wavelength bends a different amount allowing white light to wavelength bends a different amount allowing white light to separate into it’s various colors separate into it’s various colors ROYGBIV.ROYGBIV.

ULTRAVIOLET RAYSULTRAVIOLET RAYS Shorter wavelength and higher frequency than Shorter wavelength and higher frequency than

visible lightvisible light Carry Carry more energymore energy than visible light than visible light Used to Used to kill bacteria.kill bacteria. (Sterilization of equipment) (Sterilization of equipment) Causes your skin to produce Causes your skin to produce vitamin Dvitamin D (good for (good for

teeth and bones)teeth and bones) Too much can cause skin cancer. Too much can cause skin cancer. Use sun block to protect against (UV rays)Use sun block to protect against (UV rays)

X- RAYSX- RAYS Shorter wavelength and higher frequency Shorter wavelength and higher frequency

than UV-raysthan UV-rays Carry a great amount of energyCarry a great amount of energy Can penetrate most matter.Can penetrate most matter. Bones and teethBones and teeth absorbabsorb x-rays. (The light x-rays. (The light

part of an x-ray image indicates a place part of an x-ray image indicates a place where the x-ray was absorbed)where the x-ray was absorbed)

Too much exposure Too much exposure can cause cancercan cause cancer (lead vest at dentist protects organs (lead vest at dentist protects organs

from unnecessary exposure)from unnecessary exposure)

Used by Used by engineersengineers to check for to check for tiny cracks in structures.tiny cracks in structures.

The rays pass through the cracks The rays pass through the cracks and the cracks appear dark on film.and the cracks appear dark on film.

GAMMA RAYSGAMMA RAYS Shorter wavelength and higher Shorter wavelength and higher

frequency than X-raysfrequency than X-rays Carry the Carry the greatest amount of energygreatest amount of energy

and penetrate the most.and penetrate the most. Used in Used in radiation treatmentradiation treatment to kill to kill

cancer cells.cancer cells. Can be very harmful if not used Can be very harmful if not used

correctly.correctly.

SUMMARYSUMMARY All electromagnetic wavesAll electromagnetic waves travel at the travel at the

same speed. (300,000,000 meters/second same speed. (300,000,000 meters/second in a vacuum.in a vacuum.

They all have different They all have different wavelengthwavelength and and different different frequencies.frequencies. Long wavelength-Long wavelength-lowest frequencylowest frequency Short wavelengthShort wavelength highest frequency highest frequency The higher the frequency the The higher the frequency the higher the higher the

energy.energy.

Light as ParticleLight as Particle

Light also behaves like a particleLight also behaves like a particle This particle is called This particle is called a photona photon The energy of a photon is dependent on The energy of a photon is dependent on

its frequencyits frequency A photon is like a little packet (quantum) of A photon is like a little packet (quantum) of

electromagnetic energy.electromagnetic energy.

Where do photons come from?Where do photons come from?

Photons come from atomsPhotons come from atoms Atoms absorb and emit photons at certain Atoms absorb and emit photons at certain

specific frequencies.specific frequencies. This is because the electrons can only be This is because the electrons can only be

in certain orbitals.in certain orbitals.

Atomic EmissionAtomic Emission

e-

This means…This means…

Atoms of an specific element absorb or Atoms of an specific element absorb or emit only specific frequencies of emit only specific frequencies of electromagnetic energy.electromagnetic energy.

This is called the absorption or emission This is called the absorption or emission spectrum. spectrum.

Using this in astronomyUsing this in astronomy

Astronomers can use these spectra to Astronomers can use these spectra to determine the composition of stars and determine the composition of stars and planets and other stellar objects.planets and other stellar objects.

The Sun’s Emission SpectrumThe Sun’s Emission Spectrum

Wave particle dualityWave particle duality

SUMMARYSUMMARY Light can also behave light particles Light can also behave light particles

called photons.called photons. These photons come from the electrons These photons come from the electrons

in atoms.in atoms. Because electrons can only be in Because electrons can only be in

specific orbitals, only specific specific orbitals, only specific frequencies are emitted or absorbed.frequencies are emitted or absorbed.

These specific frequencies (spectrum) These specific frequencies (spectrum) can be used to determine what elements can be used to determine what elements are present.are present.

Spectroscopy

Using wavelength/frequency to determine composition

Can use light emitted emission

Or light absorbed absorption

Light and temperature

Electromagnetic spectrum also tells temperature

More heat = more energy More energy = higher frequency Higher frequency = More heat

Thermal radiation

electromagnetic radiation emitted an object because of the object's temperature

Temperature of Stars

Gamma and x-ray emitters are hottest Blue stars are hotter Red stars are colder Radio emitters are coldest

Kelvin

Kelvin is the astronomer’s temperature scale

Abbreviation : K K= ºC + 273 So… 0K = -273ºC

What’s colder than cold? Absolute zero! 0K Absolutely NO atomic motion

None Nada Zip Zilch No not even a little.

Deep space is just a few degrees above absolute zero

Summary

Spectroscopy is using electromagnetic radiation to determine composition

Thermal radiation is electromagnetic radiation emitted due to temperature

This can be used to determine the temperature and composition of stellar objects

Absolute zero (0K) is no atomic motion

Manipulating Light

Manipulating, or working with, light is what astronomers do

We have two main ways to manipulate light: Refraction Reflection

RefractionRefraction

Changing the direction of light as it passes Changing the direction of light as it passes through some mediumthrough some medium

For example:For example: Straw in drinkStraw in drink lenseslenses

RefractionRefraction

Refracting TelescopeRefracting Telescope

Focus light through a series of lensesFocus light through a series of lenses Like what Galileo usedLike what Galileo used

ReflectionReflection

Redirection of electromagnetic radiationRedirection of electromagnetic radiation Can be done with more than just visible Can be done with more than just visible

light!light!

Reflecting TelescopesReflecting Telescopes

Focus light through a series of mirrorsFocus light through a series of mirrors Invented by Isaac NewtonInvented by Isaac Newton

Reflecting TelescopesReflecting Telescopes

Allow for larger telescopesAllow for larger telescopes

English astronomer William Herschel built this huge reflecting telescope in the late 1700s.

Radio TelescopesRadio Telescopes Work like visible light reflector scopesWork like visible light reflector scopes Use the radio frequenciesUse the radio frequencies

Radio TelescopesRadio Telescopes

Radio AstronomyRadio Astronomy

Cassiopeia in visible lightCassiopeia in visible light Cassiopeia A in radioCassiopeia A in radio

Photoelectric EffectPhotoelectric Effect

Light hitting an object causes electrons to Light hitting an object causes electrons to be releasedbe released

Must be correct wavelength/frequencyMust be correct wavelength/frequency (amplitude not important)(amplitude not important)

Photoelectric effect in astronomyPhotoelectric effect in astronomy

Solar panels for spacecraftSolar panels for spacecraft Digital photographyDigital photography

Photography in AstronomyPhotography in Astronomy

As important a development as the As important a development as the telescopetelescope

Allows for sharing of informationAllows for sharing of information Allows for precise measurementsAllows for precise measurements