# Properties of Light

Urban and Civil

Nov 16, 2013 (4 years and 7 months ago)

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Information from the Cosmos

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
.

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
---

-
rays, gamma rays, cosmic rays,

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
.

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 wave?

Is it a particle?

It
is

neither,

but it’s

like

both

The
wave model

of light is illustrated by the
properties of
reflection, refraction,
diffraction, interference,

and

polarization
.

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

Waves: Standard Dimensions

In physics, waves are described by a few standard
dimensions.

Frequency
f

= how often wave crest passes,

longer wavelength means lower frequency

v

= f

x

=
Wavelength

=
㴠⁬e湧瑨t潦o潮e=cyc汥
=
Amplitude
A
= height of wave

above “rest position”

Velocity
v
= speed of wave

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

10
3

Hz = kHz (AM radio frequencies)

10
6

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

Types of waves

Transverse waves
:
the motion of the medium is at right

angles to the direction in which the wave travels.

Longitudinal waves
:
the particles in the medium move along the

direction of the wave;

travel in solids, liquids, and gases.

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.

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

a way to transfer of energy in the form of a wave

Light:

another name for

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

2.
Assume you are 100 km (~60 miles) from the
station transmitter. Calculate how long it takes

Wave speed = frequency x wavelength

Speed of light (radio waves) = c = 3x 10
8
m/sec

Distance = speed x time

x10
3

x10
6

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.

Like charges repel one another.

Unlike charges attract.

Protons

positively charged

Electrons

negatively charged

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

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.

Electric field strength
proportional to 1/r
2
.

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

=
=
=
=

=

f

=
=

so,

E

=
=
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.

Effect

cannot
be explained by ray model of light.

Sharp
-
edged

Fuzzy

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

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.

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
-

-
rays.
How much of each depends on the temperature of the

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
by the formula:

F = 9/5 C + 32

C = 5/9(F
-

32).

ABSOLUTE
SCALE K AND
°
C

Celsius
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

Temperature
Scale

Hydrogen
fuses

Water
boils

Water
freezes

All
molecular
motion
stops

Fahrenheit

18,000,032
o
F

212
o
F

32
o
F

-
459
o
F

Celsius

10,000,000
o
C

100
o
C

0
o
C

-
273
o
C

Kelvin

10,000,273 K

273 K

373 K

0 K

Planck Spectrum

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 4
th

power of its absolute temperature T.

Consider an idealized object that absorbs
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.

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.

:

the distribution of
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,
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

Type of
Wavelength
Range (
nm)
Objects at this
Temperature
Typical Sources
Gamma rays
Less than
0.01
More than
10
8
K
No astronomical sources this
hot
; some produced in nuclear
reactions.
X rays
0.01

– 20
10
6
– 10
7
K
Gas in clusters of galaxies
;
supernova remnants; solar
corona.
Ultraviolet
20-400
10
5
– 10
6
K
Supernova remnants
; very
hot stars.
Visible
400-700
10
3
– 10
5
K
Stars
Infrared
10
3
– 10
6
10 – 10
3
K
Cool clouds of dust and gas,
planets, satellites
More than
10
6
Less than 1 K
No astronomical objects this
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 4
th

power of its absolute
temperature.

This is known as the
Stefan
-
Boltzmann law
,

E =

T
4

where
E

stands for the total energy
and

=

=
Problem

-

Stefan
-
Boltzmann Law

E
T

=

T
4

The average surface
temperature of the Sun
If the Sun were twice as hot,

2 T = 2 x 5800 K

= 11,600 K,
how much more energy
does now?

Sun would be
2
4

or
16 times

more than now.

E
2T

=

=
⠲吩
4

=

=
⠲(
4

T
4

= (2)
4
(

=
T
4
)

= 16 (

=
T
4

)

= 16
E
T

Electromagnetic Spectrum

Electromagnetic Energy from the Sun

Why Do We Need Space Telescopes?

Opacity of the Atmosphere

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)

Half
-
Absorption Altitude (km)