On the nature of light - Waves

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16 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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On the nature of light
-

Waves


“Is light a wave or a particle?”



This issue was put to rest by James Clark
Maxwell (~
1873
). He studied electricity
and magnetism and the nature of waves.


On the nature of light
-

Waves


Happy Birthday Sasha


On the nature of light
-

Waves


Wave

-

a disturbance that travels from one
point to another. It is a way in which energy is
transmitted from place to place without
physical movement of material from one place
to another.

On the nature of light
-

Waves

On the nature of light
-

Waves


How are waves quantified?


Crest and trough


Amplitude

-

maximum excursion from its
undisturbed or relaxed position.


Waves travel at a speed, v. The number of crests
that pass at a specific point in space is called a
wave’s frequency or f, and is recorded in units of
Hertz.


Period

-

the time it takes for one complete cycle,
measured in seconds. This is known as P.


Wavelength

-

the distance a wave travels during one
complete oscillation.


f = 1/P Wavelength(

)

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On the nature of light
-

Waves



On the nature of light
-

Waves


Waves:



Transverse wave

-

movement or
displacement is perpendicular to the
direction of the wave.



Longitudinal wave

-

movement or
displacement is parallel to the direction of
the wave.


On the nature of light
-

Waves


“Is light a wave or a particle?”



The nature of light is that it is electromagnetic.


Electricity and magnetism are two parts of the same
electromagnetic force.



What is electricity?


A stream of electrically charged particles.



On the nature of light
-

Waves



What is an electrically charged particle?



These are particles such as electrons and protons. A
net imbalance in one charge produces an electrical
charge. Atoms gain or lose electrons and become
ionized.



Electric force

-

push and pull between electrically
charged particles.


On the nature of light
-

Waves


Electric field

-

the force exerted between
charged particles. Like charges repel and
opposite attract.


This is how electrical force is transmitted through
space.


The strength of the electric field decreases with
increasing distance from a charge, according to the
inverse
-
square law. By means of an electric field a
particle’s presence is felt by other particles.



On the nature of light
-

Waves


When a charged particle accelerates, change in the
electric field moves outward at the speed of light.


F exerted on a charged particle = charge of the
particle x the strength of the electric field.


Because an electric field points away from a
positively charged particles (or towards a negatively
charged particle), F that a second charge feels is
either directly towards or away from the first
charged particle.

On the nature of light
-

Waves


An electric field is always accompanied by a
magnetic field.



Magnetic force

-

force between electrically
charged particles arising from their motion.
This force is also exerted on moving electric
charges.


Disturbance produced by moving a charged particle
that consists of vibrating electric and magnetic
fields, always oriented perpendicular to one another
and moving together in space.

On the nature of light
-

Waves


Maxwell discovered that a change in the motion
of a charged particle causes a changing electric
field, which causes a changing magnetic field,
which causes a changing electric field …



Thus, accelerating charged particles give rise
to oscillating electric and magnetic fields that
move out in all directions or an electromagnetic
wave.

On the nature of light
-

Waves

On the nature of light
-

Waves


As discussed earlier, light is
electromagnetic in nature, it is
electromagnetic energy.

On the nature of light
-

Waves


So, information about a particle’s state of
motion is transmitted through space via a
changing electric field or magnetic field.



The disturbance that is produced in the
particle’s electric and magnetic field
travels through space as a wave.


On the nature of light
-

Waves


This means that energy produced in a star, in
the form of electromagnetic radiation, moves
outward in all directions as a wave or light.



The electromagnetic ripples travel outward as
waves. Small charged particles, either in our
eyes or in equipment, respond to the field
changes or waves by vibrating in tune with the
received radiation or wave.


THUS WE CAN SEE LIGHT!

On the nature of light
-

Spectrum


Light that is spread out, without any
interactions with particles, according to
wavelength is known as a spectrum of light.
The entire range of different wavelengths of
light is known as electromagnetic spectrum.



Wavelengths are measured in nm, µm, mm,
and cm.

On the nature of light
-

Spectrum

On the nature of light
-

Spectrum

On the nature of light
-

Spectrum


Because light is electromagnetic in
nature, when an object is heated it will
produce radiation at higher and higher
frequencies, shifting from red to blue.



We will explore this issue in more detail a
bit further down the lecture series.

On the nature of light
-

Particle


Some problems with describing light
only

as a
wave.


Basically the way electrons become excited and
release energy occurs in a specific fashion. The
amount of light energy absorbed or emitted must
correspond
precisely

to the energy difference
between two orbitals or electrons. Thus, energy is
absorbed or emitted in distinct packets known as
photons.

On the nature of light
-

Particle


Photon

-

particles of light or energy of
light; packets of electromagnetic
radiation each carrying a specific amount
of energy moving at the speed of light,
thus they are massless (quantum
mechanical approach to light).



On the nature of light
-

Particle



This description of light is related to the wave
description by a relationship between the
energy of a photon and the frequency or
wavelength of the wave.

E = hf (h =
6.63
x
10
-
34

J/s
-

Plank’s Constant
)


The higher the f of electromagnetic wave, the
greater the energy carried by each photon.

On the nature of light
-

Particle

On the nature of light
-

Spectroscopy


The analysis of the ways in which matter emits
and absorbs radiation is
spectroscopy
.


Atoms can only have discrete energies. They
can be at rest or excited.


The lowest
-
possible energy state is known as
the ground state. Allowed states with energies
lying above the ground are known as excited
states.


Ground state

-

electrons are not lost or gained, they
don’t move orbitals. If they move out of this state,
energy is produced.

On the nature of light
-

Spectroscopy


Once an atom is in an excited state it will
decay

from that state to a lower
-
lying energy state by
getting rid of some of the extra energy.


The process of loosing energy occurs at once, it
is not a gradual process. Thus, a photon is
released that contains exactly the amount of
energy lost by that atom as it jumps states. This
process is known as
emission
.


The energy state or level that an atom is within
determines the wavelength of radiation or light that
can be emitted (or absorbed).

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy


The bright, single
-
colored feature in the
spectrum is referred to as an
emission
line
.



The dark feature seen in the spectrum is
termed an
absorption line
.


Absorption is when an atom captures the
energy of a passing photon.

On the nature of light
-

Spectroscopy


Kirchhoff’s Laws

-



1
. A luminous solid or liquid, or a sufficiently dense gas, emits
light of all wavelengths and so produces a continuous spectrum
of radiation.


2
. A low
-
density hot gas emits light whose spectrum consists of
a series of bright emission lines. These lines are characteristic
of the chemical composition of the gas.


3
. A cool thin gas absorbs certain wavelengths from a
continuous spectrum, leaving dark absorption lines in their
place superimposed on the continuous spectrum. These lines
are characteristic of the composition of the intervening gas,
they occur at precisely the same wavelengths as the emission
lines produced by that gas at higher temperatures.

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy


Temperature is the measure of the energetic
nature of particles. The more energetic or the
more they move, the hotter the temperature, or
the slower the colder.



When material is dense enough for atoms to
move other atoms they will emit light simply
because of their temperature. This sort of

radiation
” is termed
thermal radiation
.

On the nature of light
-

Spectroscopy


The radiation from an object will change as it
heats or cools because the movement of atoms
changes.



When an object emits thermal radiation it is
also luminous or has
luminosity
. The hotter an
object gets, the
bluer

the radiation or light will
become.



Luminosity

-

the amount of energy radiated, or
Watts.

On the nature of light
-

Spectroscopy


Why blue?


Objects that emit radiation in an area
where it is also absorbed, in equal
proportions, is said to be a blackbody.
Thus the thermal radiation emitted is
equal to what is absorbed.



Blackbody spectrum or Planck Spectrum

On the nature of light
-

Spectroscopy


Why blue?


Plank spectrum shows that the increase in
luminosity is proportional to the fourth power of the
temperature or luminosity


T
4

(Stefan’s law).


Stafan’s law states F =


T
4



F = flux,




= Stefan
-
Boltzmann constant (
5.67
x
10
-
8
W/(m
2
K
4
)



Thus, hotter means more luminous!

On the nature of light
-

Spectroscopy


Why blue?


The total amount of light emitted is = the flux times
by the surface area of the filament.



100W = A x


T
4

solving A = 100W/


T
4

A = 4.5 x 10
-
5

m
2


Basically slight changes in temperature equate
to LARGE changes in brightness.

On the nature of light
-

Spectroscopy


Why blue?


As temperature increases, the peak of the Plank
spectrum shifts towards shorter wavelength,
meaning that the average energy of the photon gets
shorter.



We can use Wien’s law to determine that the peak
wavelength of a blackbody is inversely proportional
to its temperature. Thus, we can use this law to
calculate the temperature of an object.


peak

= 2,9000 µmK/T

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy

On the nature of light
-

Spectroscopy


Brightness

-

the amount of
electromagnetic radiation or light that
arrives at some location. Thus,
brightness is a measure of how much
light falls per square meter per second.


Twice as far = one fourth as bright

On the nature of light
-

Spectroscopy


We can use what we learned above to
determine how hot an object in space is or the
distance to that object.



Finally, in the case of planets, some light is
reflected, known as
albedo
, and some of the
energy is absorbed (
1
-

albedo). Thus, the
amount of energy emitted or absorbed by a
planet can be calculated.

On the nature of light
-

Speed


In the 1670’s
Ole Romer

was studying the
moons of Jupiter. He discovered that at
the farthest distance between the Earth
and Jupiter the moons of Jupiter appear
16.5 minutes later than predictions.
When the Earth was at the closest to
Jupiter, then moons appearance made up
the lost time.


2.99792458 x 10
8

m/s in vacuum

On the nature of light
-

Speed


Maxwell’s research discovered a “flaw” in
Newtonian physics.


The ball in the car issue
”.


The laws of Newtonian physics that govern the
above issue however, do not apply to the speed
of light.



Enter our hero, stage right;
Albert Einstein
.


Either light is a wave that does not move or Newton
was wrong when it comes to light.



On the nature of light
-

Speed


Special relativity
: Einstein focused his
attention onto events. In relativity, an
event

is a
particular location in
space and time
.


Snap your fingers while a car moves in front of you.
Snap again, time has gone by and you have not
moved. To the person in the car, according to
Newton, only time has gone by, they are still in the
car. But to the person watching the car both time
and distance have changed with the second snap.

On the nature of light
-

Speed


But…the passage of time, at the speed of
light, actually depends on the observer’s
frame of reference. Einstein discovered
space
-
time
, four
-
dimensional universe.


Time between two events is t
2
-
t
1

= 2l/c to a
stationary observer. But if the observer,
who is also stationary in perspective, is
actually moving…

On the nature of light
-

Speed


Remember, Newtonian physics applies for
objects moving much, much slower than c.


Implications of relativity:


1
. Mass and energy are actually two manifestations
of the same thing.


2
. C is the ultimate speed


3
. Time passes more slowly in a moving reference
frame.


4
. At the same time, is a relative concept.


5
. An object in motion is shorter than it is at rest.