Download Light and Matter: Decoding messages from space Light and Matter

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 ?
1
The nature of light
• 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
• Newton showed white light is made up of
many different colours
2
The nature of light
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”
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
3
Light as a wave
Light is an electromagnetic wave
• A light wave is a vibration of electric and magnetic fields
• Light interacts with charged particles through these electric
and magnetic fields
• Note: Light waves do not need a medium to propagate: can
propagate through vacuum
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
4
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
5
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
6
The electromagnetic spectrum
– Human eyes cannot see most forms of light.
– Visible light is only a small part of the
electromagnetic spectrum
– The entire range of wavelengths (frequencies)
of light is known as the electromagnetic
spectrum.
The electromagnetic spectrum
7
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.
8
Interactions of Light with Matter
• 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)
We see objects that emit light directly
We see others by light reflecting off these objects.
Thought Question
Why is a rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
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Thought Question
Why is a rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
The structure of matter
Atoms 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
10
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)
11
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 its
fingerprints in the light
• Spectroscopy is the process of dispersing light into its
spectrum (different wavelengths)
12
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 bulb
spans all visible wavelengths, without interruption
• Continuous spectra are observed from hot, dense
objects due to the motion and collisions of particles
13
Thermal Radiation
• Nearly all large or dense objects emit thermal radiation,
including stars, planets, you…
• The radiation emitted by an opaque object is called
blackbody radiation
• An blackbody’s continuous thermal radiation spectrum
depends 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
14
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.
15
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
Chemical Fingerprints in Light
• Each type of atom, ion
and molecule has a
unique ladder of
energy levels that
electrons can occupy
• The only allowed
changes in energy are
those corresponding to
a transition of an
electron between
energy levels
Not Allowed
Allowed
16
Chemical Fingerprints in Light
• Each transition of an
electron between
energy levels
corresponds to a
unique photon energy,
frequency, and
wavelength
• Downward transitions
produce a unique
pattern of emission
lines for each
atom/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
17
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 of
absorption or emission lines
• Observing the fingerprints in a spectrum tells us which
kinds of atoms are present
18
Spectral Fingerprints of Molecules
• Molecules have additional energy levels because
they can vibrate and rotate
• The large numbers of vibrational and rotational
energy levels can make the spectra of molecules
very complicated
• Many of these molecular transitions are in the
infrared part of the spectrum
The Solar Spectrum
19
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 the
wavelengths 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 the
wavelengths 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 Effect for Light
Doppler shift tells us ONLY about the part of an object’s motion
toward or away from us:
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Spectrum of a Rotating Object
• Different Doppler shifts from different sides of
a rotating object spread out its spectral lines
• Spectral lines are wider when an object rotates
faster
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.
Activity
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?
A
B
C
D E
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Summary
• What is the structure of matter?
– Matter is made of atoms, which consist of a
nucleus of protons and neutrons surrounded by
a cloud of electrons
• What are the phases of matter?
– Adding heat to a substance changes its phase
by breaking chemical bonds.
– As temperature rises, a substance transforms
from a solid to a liquid to a gas, then the
molecules 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 correspond
to particular energy levels.
– Atoms gain and lose energy only in amount
corresponding to particular changes in energy
levels.
24
Summary
• What is light ?
– Light can behave like either a wave or a particle.
– A light wave is a vibration of electric and magnetic
fields. It does not need a medium to propagate.
– The wavelength/frequency of light determines
colour.
– 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 known
as the electromagnetic spectrum.
Summary
• How does light interact with matter?
– Matter can emit light, absorb light, transmit
light, and reflect (or scatter) light.
– Interactions between light and matter
determine the appearance of everything we
see.
25
Summary
• What are the three basic type of spectra?
– Continuous spectrum, emission line
spectrum, absorption line spectrum
• How does light tell us what things are
made of ?
– Each atom has a unique fingerprint.
– We can determine which atoms something is
made of by looking for their fingerprints in
the 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
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