Download Properties of Light

Radiation
Information from the Cosmos
Radiation,Waves, & Information
• Most of the information around
us gets to us in waves.
• Sound energy that travels to
our ears is in one form of wave.
• Light is energy that comes
to our eyes if the form of
another type of wave.
• Energy (information) that is
transferred from place to
place in the form of a wave is
called RADIATION.
Information from the Cosmos
• Until recently, our knowledge of the universe
was obtained only by studying the visible
light that happened to arrive on Earth.
• Since the 1930’s, possible to study other types
of radiation and particles --– radio waves, X-rays, gamma rays, cosmic rays,
neutrinos, and gravitational radiation.
• To understand the methods used to study the
cosmos, we must understand the basic nature
and behavior of light.
So, what is light?
• The particle or ray model of light is illustrated
by the properties of reflection and refraction.
• The wave model of light is illustrated by the
properties of reflection, refraction,
diffraction, interference, and polarization.
• But there are problems:
if light is a wave, and waves need a
“medium” such as air or water to carry
them, then how can light travel through
empty space?
• The solution was to decide that light was
neither a wave nor a particle, but something
else which sometimes behaved like them.
Is it a particle?
Is it a wave?
It is neither,
but it’s
like both
What is a Wave?
• Wave motion is NOT a mechanical phenomenon because
a wave is not a material object but a form.
– It cannot be assigned a mass, and
the concept of acceleration cannot be applied to a wave.
– The motion of a wave is vastly different from
the motion of the medium in which it travels.
In fact, a wave can exist without any movement of matter at all!
• So, what is a wave? It is a pattern or form that moves.
• It can be a
– deformation of a material object
(music string or waves on the surface of a body of water)
OR
– pattern in a field
(light or radio waves).
Waves: Standard Dimensions
In physics, waves are described by a few standard
dimensions.
Wavelength  = length of one cycle
Amplitude A= height of wave
above “rest position”
Frequency f = how often wave crest passes,
longer wavelength means lower frequency
Velocity v= speed of wave
v=f
x

Frequency and Period
Frequency: how often a vibration (cycle, repetition)
occurs in some interval of time,
# vibrations (or cycles) per unit time.
units are Hertz (Hz)
1-Hz = 1 vibration/sec = 1 cycle/sec
103 Hz = kHz (AM radio frequencies)
106 Hz = MHz (FM radio frequencies)
Period: the time to complete one vibration (or cycle),
the inverse of the frequency
period = 1 / frequency OR frequency = 1 / period
Wave Speed
• The speed of some waves depends on the
medium through which the wave travels.
– Sound waves travel at speeds of
330 - 350 m/s in air,
and about four times as fast in water.
• The speed of the wave is related to the
frequency and wavelength of the wave.
Wave speed = frequency x wavelength
Motion of Waves
Is there a relationship between
the motion of the wave through space
and
the motion of the medium that a wave moves in?
Wave Types
• Two types of waves
–transverse
–longitudinal
Cheerleader demo
Types of waves
Transverse waves: the motion of the medium is at right
angles to the direction in which the wave travels.
Examples: stretched strings of musical instruments,
waves on the surfaces of liquids,
some of the waves produced in earthquakes.
Although they require no “medium” to travel,
electromagnetic waves are also transverse waves.
Longitudinal waves: the particles in the medium move along the
direction of the wave;
travel in solids, liquids, and gases.
Examples: sound waves,
some of the waves produced in earthquakes.
Do waves travel through
empty space?
What if there is no medium to move in?
Can any waves travel through empty space?
If so, which ones?
Light as a Wave
• Light is a type of radiation;
it is a type of wave that travels through space.
• Light waves are fundamentally different from
many other waves that travel only through
material media (sound or water waves).
• Light waves require NO material medium to
travel from place to place.
• The wave speed of all types of light in a vacuum
is called the speed of light, c.
c = 300,000 km/sec
Terminology
• Radiation:
a way to transfer of energy in the form of a wave
• Light:
another name for electromagnetic radiation
• Electromagnetic (EM) radiation:
Also known as light, transfers energy and
information from one place to another
(in form of coupled electric and magnetic waves)
• Visible light:
the range of electromagnetic radiation that
the human eyes perceive as visible
Group Question
1. Determine the wavelength of your group’s
favorite radio station.
2. Assume you are 100 km (~60 miles) from the
station transmitter. Calculate how long it takes
for the radio waves to arrive at your location
from the radio station transmitter.
Wave speed = frequency x wavelength
Speed of light (radio waves) = c = 3x 108m/sec
Distance = speed x time
x103 Hz (AM radio frequencies)
x106 Hz (FM radio frequencies)
Creating Electromagnetic Waves
•All matter is made up of atoms.
•Atoms are, in turn, made up of smaller particles:
protons, electrons, and neutrons.
•Two of the elementary particles that make up
atoms possess a property described as
electrical charge.
•The charges on each are equal and opposite.
electron: - charge
proton: + charge
Charged Particle Interactions
Any electrically charged object exerts a force on
other charged objects.
Electrons
negatively charged
Protons
positively charged
Like charges repel one another.
Unlike charges attract.
Electrical Force
• Electrical force:
– is a universal force
(every charged particle affects every other charged particle)
– may be attractive or repulsive force
– is always directed along the line
connecting two charges
– depends on the product of the two charges
– depends on the distance between
the two charges squared
• (obeys the “inverse square rule”)
• Today, physicists describe electric forces in
terms of an electrical field produced by the
presence of electrical charge.
Charged Particles and Electric
Fields Electric field strength
An electric field
extends outward in
all directions from
any positively
charged particle.
If a charged particle moves,
its electric field changes.
The resulting disturbance
travels through space as a
wave.
proportional to 1/r2 .
Magnetic Fields
• If an electric field changes with time
(let’s say the source charge wiggles),
then a magnetic field is created,
coupled to the time-variant electric field.
• Magnetic fields influence behavior
of magnetized objects.
– Earth’s magnetic field causes
compass needles to point N
– bar magnets
– electromagnets
Electromagnetism
Electric and magnetic fields do not exist
as independent entities.
They are different aspects of a single phenomenon:
Electromagnetism (EMR)
Together, they constitute an electromagnetic wave that carries
energy and information from one part of the universe to another.
Frequency and Energy
Light waves carry energy (E) across space.
The energy is related to the frequency of
the light wave by
E = hf
where h = Planck’s constant
Recall that wave speed relates frequency and wavelength:
v = f
and for light,
so,
E f
c = f
or E  1/
Creating and Detecting Light
• Light is created by the
motion of charged particles.
• Matter is made up of atoms, which are
in turn made up of charged particles.
• Motions of these charged particles
create light.
– Not just the light we detect with our eyes,
but at all wavelengths (or frequencies).
Electromagnetic Spectrum
Properties of Light
•
•
•
•
•
•
Polarization
Reflection
Refraction
Dispersion
Diffraction
Interference
Properties of Light:
Reflection and Refraction
• An isolated light beam travels in a straight line.
• Light can change directions under certain
conditions:
• Reflection from a surface,
– mirrors, objects
• Refraction (or bending of a ray of light) as the
ray travels from one transparent medium to
another.
– pencil in a clear glass of water
– light through a piece of glass
Properties of Light: Dispersion
•Electromagnetic waves interact with the charged particles in matter
and travel more slowly in transparent media than in a vacuum.
•The change in speed of the light wave causes the wave to refract.
•Since the speed of an EM wave in a medium changes with
wavelength, the amount of refraction depends on the wavelength.
•This effect is called dispersion.
Visible Light
• Prism will separate light into its components
• Composed of 7 hues (Roy G. Biv), known as its
spectrum
–
–
–
–
–
–
–
Red (~ 700 nm or 7000 Å)
Orange
Yellow
Green
Blue
Indigo
Violet (~ 400 nm or 4000 Å)
• Color determined by its frequency
(or, equivalently, its wavelength)
Visible Spectrum
Red
Orange Yellow
Green
Blue
Violet
Properties of Light: Diffraction
• Diffraction is the bending of a wave as it passes
through a hole or around an obstacle.
– If light consists of parallel rays, they would travel
through a small pinhole and make a small, bright
spot on a nearby screen.
Sharp-edged
shadow
Fuzzy
shadow
Effect cannot be explained by ray model of light.
Diffraction of Waves
• Actually observe a spot larger than the pinhole and
varying in brightness.
– The pinhole somehow affects the light that passes through it.
• Diffraction is proportional to the ratio of wavelength
to width of gap.
– The longer the wavelength and/or the smaller the
gap, the greater the angle through which the wave is
diffracted.
Fuzzy
shadow
Properties of Light:
Interference and Superposition
• What happens if two waves run into each other?
• Waves can interact and combine with each other,
resulting in a composite form.
• Interference is the interaction of the two waves.
– reinforcing interaction = constructive interference
– canceling interaction = destructive interference
• Superposition is the method used to model the
composite form of the resulting wave.
Interference of Waves
Interference: ability of two or more waves to reinforce
or cancel each other.
Constructive interference
occurs when two wave
motions reinforce each
other, resulting in a wave of
greater amplitude.
Destructive interference
occurs when two waves
exactly cancel, so that no
net motion remains.
Radiation and Temperature
• What determines the type of electromagnetic radiation
emitted by the Sun, stars, and other astronomical
objects? Temperature
• Electromagnetic radiation is emitted when electric
charges accelerate, changing either the speed or the
direction of their motion.
• The hotter the object, the faster the atoms move in the
object, jostling one another, colliding with more
electrons, changing their motions with each collision.
• Each collision results in the emission of electromagnetic
radiation- radio, infrared, visible, ultraviolet, x-rays.
How much of each depends on the temperature of the
object producing the radiation.
Measuring Temperature
• Atoms and molecules that make up matter
are in constant random motion.
• Temperature is a direct measure of this
internal motion.
– The higher the temperature,
the faster (on average) the random motion
of particles in matter.
– Temperature of an object represents the average
thermal energy of particles
that
make up that object.
TWO MAJOR
SCALES °F and °C
• Fahrenheit scale based
on temperature that salt
water freezes 0°F
(lower than pure water).
• Related to Celsius
(or Centigrade)
by the formula:
F = 9/5 C + 32
C = 5/9(F - 32).
ABSOLUTE
SCALE
K AND °C
• Celsius
(originally Centigrade)
based on freezing and
boiling point of pure water,
chosen to be 0°C and 100°C
• Kelvin based on absolute
coldest temperature
possible (absolute zero)
• Related by
K = C – 273.15
C = K + 273.15
Temperature Scales
All
Water Water molecular
boils freezes
motion
stops
Temperature
Scale
Hydrogen
fuses
Fahrenheit
18,000,032oF
212oF
32oF
-459oF
Celsius
10,000,000oC
100oC
0oC
-273oC
Kelvin
10,000,273 K
273 K
373 K
0K
Radiation Laws
• Blackbody Radiation
– Planck Spectrum
– Characteristics of Radiator
• Wien’s Law
– Relates wavelength at which a blackbody
emits its maximum energy, max, to the
temperature, T, of the blackbody.
• Stefan-Boltzmann Law
– Relates total energy emitted per second per
square meter by a blackbody, E, to the 4th
power of its absolute temperature T.
Blackbody Radiation
• Consider an idealized object that absorbs
all the electromagnetic radiation that falls
on it - called a “blackbody.”
• A blackbody absorbs all energy incident
on it and heats up until it is emitting
energy at the same rate that it absorbs
energy.
• The equilibrium temperature reached is a
function of the total energy striking the
blackbody each second.
Characteristics of Blackbody Radiation
• A blackbody with a temperature higher
than absolute zero emits some energy at
all frequencies (or wavelengths).
• A blackbody at higher temperature emits
more energy at all frequencies
(or wavelengths) than does a cooler one.
• The higher the temperature of a blackbody,
the higher the frequency (the shorter the
wavelength) at which the maximum energy
is emitted.
Blackbody Radiation
• Blackbody radiation:
the distribution of
radiation emitted by any
heated object.
• The curve peaks at a
single, well-defined
frequency and falls off to
lesser values above and
below that frequency.
The overall shape (intensity vs frequency) is characteristic
of the radiation emitted by any object, regardless of its
size, shape, composition, or temperature.
Planck Spectrum
• As an object is heated,
the radiation it emits
peaks at higher and
higher frequencies.
• Shown here are curves
corresponding to
temperatures of
300 K (room temperature),
1000 K (glow dull red),
4000 K (red hot), and
7000 K (white hot).
“Red Hot”
• As something begins to heat-up, there
probably isn’t any visible information to tell
you it is warming up.
• Once it starts to glow red, you have learned
it’s hot – don’t touch.
– Like the stove burners.
• As it continues getting hotter, it changes to
orange, then yellow, green, blue and white.
Wien’s Law
• The Sun and stars emit energy that
approximates the energy from a blackbody.
• It is possible to estimate their temperatures by
measuring the energy they emit as a function of
wavelength - that is, by measuring their color.
• The wavelength at which a blackbody emits its
maximum energy can be calculated by
 max = 3,000,000 / T
where the wavelength  max is in nanometers (10-9 m)
and the temperature T is in kelvin.
• This relationship is known as Wien’s law.
Effect of Temperature
Hotter objects are brighter and “bluer”
than cooler objects.
Getting
Warmer
Electromagnetic Radiation
Type of
Radiation
Wavelength
Range (nm)
Radiated by
Objects at this
Temperature
Typical Sources
Gamma rays
Less than
0.01
More than
108 K
X rays
0.01 – 20
106 – 107 K
Ultraviolet
20-400
105 – 106 K
Visible
400-700
103 – 105 K
No astronomical sources this
hot; some produced in nuclear
reactions.
Gas in clusters of galaxies;
supernova remnants; solar
corona.
Supernova remnants; very
hot stars.
Stars
Infrared
103 – 106
10 – 103 K
Radio
More than
106
Less than 1 K
Cool clouds of dust and gas,
planets, satellites
No astronomical objects this
cold: radio emission
produced by electrons
moving in magnetic fields
Problem - Wien’s law
• The average surface temperature of the Sun
is about 5800 K. At what wavelength is
maximum energy emitted from the Sun?
• If T = 5800 K
• and max = 3,000,000 / T ,
• then max = 3,000,000 / 5800 = 520 nm.
• 520 nm is at the middle of the visible light
portion of the electromagnetic spectrum.
• The human eye is most sensitive to the
wavelengths at which the Sun puts out the
most energy.
Stefan-Boltzmann Law
• If add up the contributions from all parts of the
E-M spectrum, obtain the total energy emitted by
a blackbody over all wavelengths.
• That total energy emitted per second per square
meter by a blackbody at temperature T
is proportional to the 4th power of its absolute
temperature.
• This is known as the Stefan-Boltzmann law,
E = T4
where E stands for the total energy
and  is a constant number.
Problem - Stefan-Boltzmann Law
ET = T4
• E2T =  (2T)4
•The average surface
temperature of the Sun
•
=  (2)4 T4
is about 5800 K.
4 ( T4 )
•
=
(2)
If the Sun were twice as hot,
2 T = 2 x 5800 K
•
= 16 ( T4 )
= 11,600 K,
•
= 16 ET
how much more energy
would it radiate than it
The energy radiated by the
does now?
Sun would be 24 or 16 times
more than now.
Electromagnetic Spectrum
Electromagnetic Energy from the Sun
Why Do We Need Space Telescopes?
Opacity of the Atmosphere
Half-Absorption Altitude (km)
• Only a small fraction of the radiation produced by astronomical
objects actually reaches our eyes because atoms and molecules in
the Earth's atmosphere absorb certain wavelengths and transmit
others.
• Opacity is proportional to the amount of radiation that is absorbed
by the atmosphere.
Wavelength (angstroms)