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Light and Matter:

Decoding messages from space

Reading: Chapters 4.1, 5.2, 5.4

Light and Matter

Questions:

• What is light ?

• What is the electromagnetic spectrum?

• How do light and matter interact ?

• What does light tell us about objects in the

universe ?

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• The warmth of sunlight tells us that light is

a form of energy: radiative energy

Units of energy: joules

• We can measure the flow of energy in light

in units of watts: 1 watt = 1 joule/s

The nature of light

The nature of light

• Newton showed white light is made up of

many different colours

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1670, Hooke, Huygens: Light behaves like a wave.

1700, Newton: Light consists of particles (corpuscles)

1800, Young, Fresnel: Showed wavelike properties of light

1873, Maxwell: Identified light as electromagnetic waves

1905, Einstein: Light consists particles of energy called “photons”

The nature of light

So is light made of particles or waves ?

Quantum Mechanics: Light behaves as both, particle and wave.

What are waves ?

• A wave is periodic motion that can carry

energy without carrying matter along

with it

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• A light wave is a vibration of electric and magnetic fields

• Light interacts with charged particles through these electricand magnetic fields

• Note: Light waves do not need a medium to propagate: canpropagate through vacuum

Light as a waveLight is an electromagnetic wave

Properties of Waves

• Wavelength is the distance between two wave peaks

• Frequency is the number of times per second that a

wave vibrates up and down

wave speed = wavelength x frequency

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Wavelength and Frequency

wavelength x frequency = speed of light = constant

Speed of light is 300,000,000 m/s

Thus lower wavelength means higher frequency and vice versa

Wavelength and Frequency

• wavelengths/frequencies are related to colours

Longer wavelengths/lower frequencies: redder light

Shorter wavelengths/higher frequencies: bluer light

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Light as a particle

• Particles of light are called photons

• Each photon has a wavelength (!) and a

frequency (f)

• The energy of a photon is proportional to

its frequency and inversely prpoportional

to its wavelength

Wavelength, Frequency, and Energy

! x f = c

! = wavelength , f = frequency

c = 3.00 x 108 m/s = speed of light

E = h x f = photon energy

h = 6.626 x 10-34 joule x s = Planck’s constant

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The electromagnetic spectrum

– Human eyes cannot see most forms of light.

– Visible light is only a small part of theelectromagnetic spectrum

– The entire range of wavelengths (frequencies)of light is known as the electromagneticspectrum.

The electromagnetic spectrum

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Thought Question

The higher the photon energy…

a) the longer its wavelength.

b) the shorter its wavelength.

c) energy is independent of wavelength.

Thought Question

The higher the photon energy…

a) the longer its wavelength.

b) the shorter its wavelength.

c) energy is independent of wavelength.

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• Emission: Energy in matter can be converted into light that

is emitted

• Absorption: Matter can absorb energy in the form of light

and convert it to another form or re-emit it

• Transmission

– Transparent objects transmit light

– Opaque objects block (absorb) light

• Reflection or Scattering: Light can bounce off objects in

one direction (reflection) or random directions (scattering)

Interactions of Light with Matter

We see objects that emit light directly

We see others by light reflecting off these objects.

Thought Question

Why is a rose red?

a) The rose absorbs red light.

b) The rose transmits red light.

c) The rose emits red light.

d) The rose reflects red light.

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Thought Question

Why is a rose red?

a) The rose absorbs red light.

b) The rose transmits red light.

c) The rose emits red light.

d) The rose reflects red light.

The structure of matterAtoms are the building blocks of matter.

Every element is made up of a different type of atom

All atoms are made of

- protons with positive charge of +1

- neutrons with 0 charge

- electrons with negative charge of -1

Atoms of different elements have a different number of protons

Chemical properties depend on number of electrons in the atom

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The structure of matter

The size of an atom is less than 1 millionth of a millimetre !

The nucleus is 100,000 times smaller in size but incredibly dense

The nucleus contains most of the mass of the atom: protons and

neutrons.

Mass of protons and neutrons is 2000 times the mass of

electrons

Atomic Terminology

• Atomic Number = # of protons in nucleus

• Atomic Mass Number = # of protons + neutrons

• Neutral atoms have no net charge: number of protons= number of electrons

• Ions are either positively charged(number of electrons < number of protons)or negatively charged(number of electrons > number of protons).

• Isotope: same # of protons but different # of neutrons.(4He, 3He)

• Molecules: consist of two or more atoms (H2O, CO2)

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Phases of matter

• Familiar phases:

– Solid (ice)

– Liquid (water)

– Gas (water vapor)

• Phases of the same material behave differently

because of differences in strength and type of

chemical bonds between atoms and molecules

Light is the cosmic messenger

• Light travels to us from all parts of the universe

• Matter in the universe interacting with light leaves itsfingerprints in the light

• Spectroscopy is the process of dispersing light into itsspectrum (different wavelengths)

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Three basic types of spectra

Spectra of astrophysical objects

are usually combinations of

three basic types:

Continuous spectrum

Absorption spectrum

Emission spectrum

Continuous (Thermal) Spectrum

• The spectrum of a common (incandescent) light bulbspans all visible wavelengths, without interruption

• Continuous spectra are observed from hot, denseobjects due to the motion and collisions of particles

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Thermal Radiation

• Nearly all large or dense objects emit thermal radiation,including stars, planets, you…

• The radiation emitted by an opaque object is calledblackbody radiation

• An blackbody’s continuous thermal radiation spectrumdepends on only one property: its temperature

Properties of Thermal Radiation

1. Stefan-Boltzman Law:

Hotter objects emit energy

than cooler objects.

2. Wien’s Law: Hotter objects

emit photons with a higher

average energy. The

wavelength of peak intensity

decreases (shifts towards blue)

as the temperature increases

Hence the thermal spectrum

can tell us the temperature of a

star

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Thought Question

Which is hotter?

a) A blue star.

b) A red star.

c) A planet that emits only infrared light.

Thought Question

Which is hotter?

a) A blue star.

b) A red star.

c) A planet that emits only infrared light.

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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

• Each type of atom, ionand molecule has aunique ladder ofenergy levels thatelectrons can occupy

• The only allowedchanges in energy arethose corresponding toa transition of anelectron betweenenergy levels

AllowedNot Allowed

Chemical Fingerprints in Light

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Chemical Fingerprints in Light

• Each transition of anelectron betweenenergy levelscorresponds to aunique photon energy,frequency, andwavelength

• Downward transitionsproduce a uniquepattern of emissionlines for eachatom/ion/molecule

Chemical Fingerprints in Light

• Because those

atoms/ions/molecules

can absorb photons with

those same energies,

upward transitions

produce a pattern of

absorption lines at the

same wavelengths

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Absorption Line Spectrum

• A cloud of gas between us and a light source can absorb

light of specific wavelengths, leaving dark absorption

lines in the continuous spectrum

Chemical Fingerprints in Light

• Each type of atom has a unique spectral fingerprint ofabsorption or emission lines

• Observing the fingerprints in a spectrum tells us whichkinds of atoms are present

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• Molecules have additional energy levels becausethey can vibrate and rotate

• The large numbers of vibrational and rotationalenergy levels can make the spectra of moleculesvery complicated

• Many of these molecular transitions are in theinfrared part of the spectrum

Spectral Fingerprints of Molecules

The Solar Spectrum

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How does light tell us the speed

of a distant object?

The Doppler Effect: The frequency of waves measured by

an observer changes if the source of the waves is moving.

Example: Change in sound of siren as ambulance passes.

Doppler Effect for Light

• We measure the Doppler Effect from shifts in thewavelengths of spectral lines

• Red shift: object moving away

• Blue shift: Object moving towards us

• The larger the shift the faster the object is moving

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Doppler Effect for Light

• We measure the Doppler Effect from shifts in thewavelengths of spectral lines

• Red shift: object moving away

• Blue shift: Object moving towards us

• The larger the shift the faster the object is moving

Doppler shift tells us ONLY about the part of an object’s motion

toward or away from us:

Doppler Effect for Light

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Spectrum of a Rotating Object

• Different Doppler shifts from different sides ofa rotating object spread out its spectral lines

• Spectral lines are wider when an object rotatesfaster

Thought Question

I measure a line in the lab at 500.7 nm.

The same line in a star has wavelength 502.8 nm.

What can I say about this star?

a) It is moving away from me.

b) It is moving toward me.

c) It has unusually long spectral lines.

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Thought Question

I measure a line in the lab at 500.7 nm.

The same line in a star has wavelength 502.8 nm.

What can I say about this star?

a) It is moving away from me.

b) It is moving toward me.

c) It has unusually long spectral lines.

A B C D E

This is a spectrum of the planet Mars

Which letter(s) labels absorption lines?

Which letter(s) labels emission lines?

What features explain why Mars appears red in colour?

What change would we observe in the spectrum when Mars

is moving away from earth in its orbit?

Activity

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Summary• What is the structure of matter?

– Matter is made of atoms, which consist of anucleus of protons and neutrons surrounded bya cloud of electrons

• What are the phases of matter?

– Adding heat to a substance changes its phaseby breaking chemical bonds.

– As temperature rises, a substance transformsfrom a solid to a liquid to a gas, then themolecules can dissociate into atoms

– Stripping of electrons from atoms (ionization)turns the substance into a plasma

Summary

• How is energy stored in atoms?

– The energies of electrons in atoms correspondto particular energy levels.

– Atoms gain and lose energy only in amountcorresponding to particular changes in energylevels.

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Summary• What is light ?

– Light can behave like either a wave or a particle.

– A light wave is a vibration of electric and magneticfields. It does not need a medium to propagate.

– The wavelength/frequency of light determinescolour.

– The speed of light is constant.

– Photons are particles of light.

• What is the electromagnetic spectrum?

– Human eyes cannot see most forms of light.

– The entire range of wavelengths of light is knownas the electromagnetic spectrum.

Summary

• How does light interact with matter?

– Matter can emit light, absorb light, transmitlight, and reflect (or scatter) light.

– Interactions between light and matterdetermine the appearance of everything wesee.

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Summary

• What are the three basic type of spectra?

– Continuous spectrum, emission linespectrum, absorption line spectrum

• How does light tell us what things aremade of ?

– Each atom has a unique fingerprint.

– We can determine which atoms something ismade of by looking for their fingerprints inthe spectrum.

Summary

• How does light tell us the temperatures of

planets and stars?

– Nearly all large or dense objects emit a

continuous spectrum that depends on

temperature.

– The spectrum of that thermal radiation tells

us the object’s temperature.

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Summary

• How does light tell us the speed of a distant object?

– The Doppler effect tells us how fast an object is

moving toward or away from us.

• Blueshift:objects moving toward us

• Redshift: objects moving away from us

• How does light tell us the rotation rate of an object ?

– The width of an object’s spectral lines can tell us how

fast it is rotating