Light and Matter
Light in Everyday Life
Our goals for learning:
• How do we experience light?
• How do light and matter interact?
The warmth of sunlight tells us that light is a form of energy.
Energy is measured in joules. We can measure the flow of
energy in light in units of watts: 1 watt = 1 joule/s
Interactions of Light
4 process:
• Emission
• Absorption
• Transmission
• Transparent objects transmit
(allow to pass) light
• Opaque objects block (absorb)
light
• Reflection or Scattering
• White light is made up of many different colors
Reflection and Scattering
Mirror reflects
light in a particular
direction
Movie screen scatters light
in all directions
Interactions of Light with Matter
Interactions between light and matter determine the appearance
of everything around us. Objects with color (e.g. a red rose)
appear that color because they absorb all the other colors and
reflect (or scatter) that one.
What is light?
• Light is a form of energy that can act either
like a wave or like a particle (with energy
and momentum) depending on its interaction
with matter
– A light wave is a vibration of electric and
magnetic fields – an electromagnetic wave
– Particles of light are bundles (quanta) of energy
called photons
Waves
• A wave is a
pattern of
motion that
can carry
energy
without
carrying
matter along
with it
Leave bobs
up and down
Properties of Waves
Eyes are sensitive to wavelength; we sense
wavelength differences as “color”.
• Wavelength is the distance between two wave peaks
• Frequency is the number of times per second that a
wave vibrates up and down (cycles per second or hertz)
wave speed = wave length x frequency
Mathematically: c = f
: the Greek letter lambda
Particles of Light
• Particles of light are called photons
• Each photon has a wavelength and a
frequency associated with it
• The Energy of a photon depends on its
frequency, E = hf
• h is a constant of nature called Planck’s
constant
Wavelength, Frequency, and Energy
 x f = c
 = wavelength, f = frequency
Visible Light: = 0.5 x 10-6 m, f = 6 x 1014 s-1 (Hz)
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
One photon of visible light carries ~4 x 10-19 joules of energy
A 100W bulb emits 2.5 x 1020 photons every second!
What have we learned?
• What is light?
– Light can behave like either a wave or a
particle
– A light wave is a vibration of electric and
magnetic fields
– Light waves have a wavelength and a
frequency
– 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.
Properties of Matter
Our goals for learning:
• What is the structure of matter?
• What are the phases of matter
• How is energy stored in atoms?
We need to know this in order to understand the end phase of a
star’s life & what’s happening inside a white dwarf or neutron
star.
What is the structure of matter?
Everything is made of atoms
Electrons have negative charge
and are almost 2000x less
massive than protons
Electron
Cloud
10-15 m
Nucleus
Atom
Neutron:
no charge
Proton:
positive charge
Volume of atom = 1,000 trillion times that of nucleus
Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons + neutrons
• Molecules: consist of two or more atoms (H2O, CO2)
Atomic Terminology
• Isotope: same # of protons but different # of neutrons.
Example: (4He, 3He)
What are the phases of matter?
• Phases:
–
–
–
–
Solid (ice)
Liquid (water)
Gas (water vapor)
Plasma (ionized gas)
• Phases of same material behave differently
because of differences in chemical bonds. By
chemical bonds we mean the electric forces
between atoms.
Phase Changes
Read from bottom to top
• Ionization: Stripping of electrons,
changing atoms into plasma
• Dissociation: Breaking of
molecules into atoms
• Evaporation: Breaking of flexible
chemical bonds, changing liquid
into gas
• Melting: Breaking of rigid
chemical bonds, changing solid
into liquid
What have we learned?
• 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
Learning from Light
Our goals for learning:
• What are the three basic types of spectra?
• How does light tell us what things are made
of?
• How does light tell us the temperatures of
planets and stars?
• How do we interpret an actual spectrum?
What are the 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. We can take a picture of a spectrum (lower bar)
or we can plot a graph of intensity versus wavelength (upper).
Continuous Spectrum
Slit in screen
• The spectrum of a common (incandescent) light
bulb spans all visible wavelengths, without
interruption
Emission Line Spectrum
Each colored “line” is an
image of the entrance slit.
• 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
Chemical Fingerprints
• Electrons in atoms
can only occupy
certain energy states
or levels
• The lowest energy
state (level 1) is the
Ground State
• Downward transitions
between energy states
produce a unique
pattern of emission
lines (E = hf = hc/)
Chemical Fingerprints
• Because those atoms
can absorb photons
with those exact
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
Energy Levels of Molecules
• Molecules have additional energy levels because
they can vibrate and rotate
• The “spring” just represents the electrical bond
between the atoms of the molecule
Energy Levels of Molecules
• The large numbers of vibrational and rotational
energy levels can make the spectra of molecules very
complicated
• Many of the energy transitions due to vibration and
rotation of molecules occur in the infrared part of the
spectrum
Thermal Radiation
• Nearly all large or dense objects emit thermal
radiation, including stars, planets, you…
• Collisions between atoms in a hot object causes
electrons to jump to higher energy levels for a
while and then drop down again to emit light. As a
result, the photons produced are intimately linked
with the temperature (average kinetic energy) in
the collisions. Radiation produced this way is
called thermal.
• An object’s thermal radiation spectrum depends
on only one property: its temperature
Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies per
unit area.
Power per sq. meter = σT4 (Stefan’s Law)
2. Hotter objects emit photons with a higher average
energy.
maxT ~ 3000 (for  in m)
(Wien’s Law)
Larger objects can
emit more total light
even if they are
cooler. For a sphere
(star), luminosity is L
= 4πR2σT4
Thought Question
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes.
c) People are too small to emit enough light for us
to see.
d) People do not contain enough radioactive
material.
Thought Question
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes. We glow in the infrared.
c) People are too small to emit enough light for us
to see.
d) People do not contain enough radioactive
material.
What have we learned?
• 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.
What have we learned?
• 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.
• How do we interpret an actual spectrum?
– By carefully studying the features in a
spectrum, we can learn a great deal about the
object that created it.
The Doppler Effect
Our goals for learning:
• How does light tell us the speed of a distant
object?
• How does light tell us the rotation rate of an
object?
How does light tell us the speed
of a distant object?
The Doppler Effect
Waves are compressed in the
direction of motion  wavelength
is decreased frequency is higher
Same thing happens
for light.
Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
• We generally measure the Doppler Effect from shifts in the
wavelengths of spectral lines
• The fractional shift is: ( - 0)/0 where 0 is the undisturbed
wavelength; this number is equal to the speed of the object
relative to that of light (V/c)
Doppler shift tells us ONLY about the part of an
object’s motion toward or away from us:
Pure radial motion –
maximum Doppler shift
Transverse motion –
no Doppler shift
Part radial, part transverse –
Doppler shift gives Vr = Vcosθ
(less than V)
θ Vr
Thought Question
I measure a spectral 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.
Thought Question
I measure a spectral 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. This is a REDSHIFT
b) It is moving toward me.
c) It has unusually long spectral lines.
And redshift = (502.8 – 500.7)/500.7 = 0.004194
Therefore the radial component of velocity = 0.004194 x c
= 1,258 km/s
Measuring
Redshift
How does light tell us the
rotation rate of an object?
Spectrum of a Rotating Object
Slower
Faster
Spectral lines are wider when an object rotates faster
What have we learned?
• 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
Telescopes: Portals of Discovery
A Selection of Major Telescopes
Chile
2 x 10m
4 x 8m
Mauna Kea, Hawaii
Hubble: 2.5m in space
X-ray telescope
in space
New Mexico
VLA:27 radio
telescopes
Eyes and Cameras:
Everyday Light Sensors
Our goals for learning:
• How does your eye form an image?
• How do we record images?
How does your eye form an
image?
The lens bends (refracts) light rays and
brings to a focus on retina.
Refraction
• Refraction is
the bending of
light when it
passes from one
substance into
another
• Your eye uses
refraction to
focus light
Focusing Light
• Refraction can cause parallel light rays to converge
to a focus
Image Formation
• The focal plane is where light from different
directions comes into focus
• The image behind a single (convex) lens is actually
upside-down!
Focusing Light
Digital cameras
detect light with
charge-coupled
devices (CCDs)
• A camera focuses light like an eye and captures the
image with a “detector”
• The electronic detectors in digital cameras are
similar to those used in modern telescopes
What have we learned?
• Tell more of the story how digital cameras started in the military and
astronomy
• How does your eye form an image?
– It uses refraction to bend parallel light rays so that they form an
image.
– The image is in focus if the focal plane is at the retina.
• How do we record images?
– Cameras focus light like your eye and record the image with a
detector.
– The detectors (CCDs or charge-coupled devices) in digital
cameras are like those used on modern telescopes; these devices
convert photons into electrons and then into an electronic image
that gets stored in a computer
Telescopes: Giant Eyes
Our goals for learning:
• What are the two most important properties
of a telescope?
• What are the two basic designs of
telescopes?
• What do astronomers do with telescopes?
What are the two most important
properties of a telescope?
1. Light-collecting area: Telescopes with a
larger collecting area can gather a greater
amount of light in a shorter time.
2. Angular resolution: Telescopes that are
larger are capable of taking images with
greater detail.
Light Collecting Area
• A telescope’s diameter tells us its lightcollecting area: Area = π(diameter/2)2
• The largest telescopes currently in use have a
diameter of about 10 meters: these are the
twin 10m telescopes of the W. M. Keck
Observatory which are owned and operated
by the California Association for Research in
Astronomy (UC and Caltech).
Bigger is better
Thought Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
Thought Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater. Fives times larger
in diameter means 25 times more area.
Angular Resolution
• The minimum
angular separation
that the telescope
can distinguish.
• At a great distance the
pair of lights will look
like one rather than two;
the light will still be
visible but we will not
be able to distinguish
that it is from two
sources.
Angular Resolution
• Ultimate limit to
resolution comes
from interference of
light waves within a
telescope.
• Larger telescopes
are capable of
greater resolution
because there’s less
interference
Angular Resolution
• The rings in this
image of a star come
from interference of
light wave.
Close-up of a star from the Hubble
Space Telescope
• This limit on angular
resolution is known as
the diffraction limit
• Angular resolution
scales as /D
 = wavelength; D = telescope diameter
What are the two basic designs
of telescopes?
• Refracting telescope: Focuses light with
lenses
• Reflecting telescope: Focuses light with
mirrors
Refracting Telescope
• Refracting
telescopes
need to be
very long,
with large,
heavy lenses
Largest lens is ~1m diameter
Reflecting Telescope
• Reflecting telescopes can have much greater
diameters; invented by Isaac Newton
• Most modern telescopes are reflectors
Designs for Reflecting Telescopes
Mirrors in Reflecting Telescopes
Twin Keck telescopes on
Mauna Kea in Hawaii
Segmented 10-meter mirror
of a Keck telescope
Hard to make really big mirrors by grinding glass. The Keck telescopes have 36
hexagonal segments computer-controlled to act like one large curved mirror.
What do astronomers do with
telescopes?
• Imaging: Taking pictures of the sky
• Spectroscopy: Breaking light into spectra
• Timing: Measuring how light output varies
with time
Imaging
• Astronomical
detectors
generally
record only
one color of
light at a time
• Several images
must be
combined to
make full-color
pictures
Imaging
• Astronomical
detectors can
record forms
of light our
eyes can’t see
• False Color is
sometimes used
to represent
different energies
of non-visible
light
Spectroscopy
Light from
only one star
enters
• A spectrograph
separates the
Diffraction
different
grating breaks
wavelengths of
light into
light before they
spectrum
hit the detector
Detector
records
spectrum
Spectroscopy
• Graphing
relative
brightness of
light at each
wavelength
shows the
details in a
spectrum
Timing
• A light curve represents a series of brightness
measurements made over a period of time
What have we learned?
• What are the two most important properties of a
telescope?
– Collecting area determines how much light a
telescope can gather
– Angular resolution is the minimum angular
separation a telescope can distinguish
• What are the two basic designs of telescopes?
– Refracting telescopes focus light with lenses
– Reflecting telescopes focus light with mirrors
– The vast majority of professional telescopes
are reflectors
What have we learned?
• What do astronomers do with telescopes?
– Imaging
– Spectroscopy
– Timing
Telescopes and the Atmosphere
Our goals for learning:
• How does Earth’s atmosphere affect
ground-based observations?
• Why do we put telescopes into space?
How does Earth’s atmosphere
affect ground-based observations?
• The best ground-based sites for
astronomical observing are
–
–
–
–
Calm (not too windy)
High (less atmosphere to see through)
Dark (far from city lights)
Dry (few cloudy nights)
Light Pollution
• Scattering of human-made light in the atmosphere
is a growing problem for astronomy
Twinkling and Turbulence
Star viewed with groundbased telescope
Same star viewed with
Hubble Space Telescope
Turbulent air flow in Earth’s atmosphere distorts
our view, causing stars to twinkle and images to blur
Adaptive Optics
A new technology that corrects for atmospheric turbulence
Without adaptive optics
With adaptive optics
How is it done? Rapidly changing the shape of a
telescope’s mirror can compensate for some of the effects
of turbulence
Calm, High, Dark, Dry
• The best
observing
sites are atop
remote
mountains
This is where
many astronomers
work.
Summit of Mauna Kea, Hawaii
Why do we put telescopes into space?
Transmission in Atmosphere
• Only radio and visible electromagnetic waves pass
easily through Earth’s atmosphere
• We need telescopes in space to observe other forms
What have learned?
• How does Earth’s atmosphere affect groundbased observations?
– Telescope sites are chosen to minimize the
problems of light pollution, atmospheric
turbulence, and bad weather.
• Why do we put telescopes into space?
– Forms of light other than radio and visible do
not pass through Earth’s atmosphere.
– Also, much sharper images are possible
because there is no turbulence.
Telescopes and Technology
Our goals for learning:
• How can we observe non-visible light?
• How can multiple telescopes work together?
• How adaptive optics work animation
How can we observe non-visible light?
• A standard
satellite dish
is essentially
a telescope
for observing
radio waves
Radio Telescopes
• A radio
telescope is
like a giant
mirror that
reflects radio
waves to a
focus
Arecibo: 300m
Infrared Telescopes
SOFIA
Spitzer
• Infrared (and ultraviolet-light) telescopes operate
like visible-light telescopes but need to be above
atmosphere to see all IR (and UV wavelengths);
examples of a UV telescope – Hubble, GALEX
X-Ray Telescopes
• X-ray
telescopes
also need to
be above the
atmosphere
Chandra
X-rays are harder
to focus …
X-Ray Telescopes
• Focusing of X-rays requires special mirrors
• Mirrors are arranged to focus X-ray photons through
grazing bounces off the surface
Gamma Ray Telescopes
• Gamma ray
telescopes also
need to be in
space
• Focusing
gamma rays is
extremely
difficult
Compton Observatory
Current Missions: SWIFT, FERMI
How can multiple telescopes
work together?
Interferometry
• Interferometry
is a technique
for linking two
or more
telescopes so
that they have
the angular
resolution of a
single large one
Interferometry
• Easiest to do
with radio
telescopes
• Now becoming
possible with
infrared and
visible-light
telescopes also
Very Large Array (VLA), Socorro, New Mexico
Concept for a 30-meter
Giant Segmented Mirror
Telescope