Photons: Properties, Measurement, and Sources, Slides of Astrophysics

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Light

Photons

The

photon

is

the

gauge

boson

of

the

electromagnetic

force.

Massless

Stable

Interacts

with

charged

particles.

Photon

velocity

depends

on

the

medium.

c

8

m/s

n

index

of

refraction

The

light

year

is

a

distance,

ly

12

km.

n

c

c

Electromagnetic

Radiation

Traditional

upper

boundaries

for

types

of

EM

radiation:

(m)

(Hz)

E

(eV)

Radio

waves

8

‐

6

Microwaves

‐

3

11

‐

3

Infrared

‐

6

14

Visible

light

‐

6

14

Ultraviolet

light

‐

8

16

2

X

rays

‐

11

19

5

Gamma

rays

(highest

energy)

Sources

of

Photons

Accelerated

charges

emit

photons.

Continuous

or

discrete

spectra

may

result

Moving charge

Emitted photon

Photons can be reabsorbed as well.

Black

Body

A

black

object

is

perfectly

absorbing.

Absorption

coefficient

is

The

distribution

is

just

due

to

emission.

An

isolated

cavity

with

a

narrow

hole

radiates

like

a

perfectly

black

body

at

the

same

temperature









I

Blackbody

Thermodynamics

Assume

the

cavity

has

particles

which

interact

with

the

wall.

Relativistic

photon

energy

Relate

to

energy

density

Apply

the

nd

law

to

the

energy.

Stefan

Boltzmann

law

Real

objects

have

a

factor

for

emissivity

4

0

)

(

T

u

T

u



pV

U

3









0

)

,

(

)

(

)

,

(

d

T

W

V

T

Vu

V

T

U

V

T

dp dT

T

p

dUdV

  

  







  

  

u T

du dT

4



lowenergy

highenergy

frequency

intensity

Blackbody

Radiation

Heated

gas

radiates

electromagnetic

energy

as

blackbody

radiation.

The

frequency

spectrum

power

is

a

function

of

temperature.

W



T

Earth

surface:

K

ºC

Sun

surface:

K

ºC

Sun

interior:

7

K

Atoms

and

Light

Atomic

electron

energy

levels

are

a

source

of

discrete

photon

energies.

Change

from

a

high

to

low

energy

state

produces

a

photon.

Atoms

can

also

absorb

a

photon

to

excite

an

electron.

Discrete

Spectrum

Each

atom

has

its

own

set

of

energy

levels.

Each

atom

generates

photons

at

specific

frequencies.

The

pattern

of

frequencies

identifies

the

atom.

helium

neon

Absorption

Lines

Ionized

gases

at

a

star’s

surface

absorb

specific

frequencies

of

light.

These

appear

as

dark

lines

in

a

star’s

spectrum.

Since

gases

ionize

at

different

temperatures,

the

appearance

of

lines

indicate

the

temperature

of

the

star.

Fluorescence

Atoms

and

molecules

can

reemit

absorbed

energy.

Fluorescence

typically

involves

three

steps.

Excitation

to

higher

energy

state.

Loss

of

energy

through

change

in

vibrational

state

Emission

of

fluorescent

photon.

10

s

S

1 S

0

10

s

10

s

X

Rays

X

rays

are

associated

with

energetic

transitions

in

atoms.

Continuous

spectra

result

from

electron

bombardment.

Discrete

spectra

result

from

electron

transitions

with

an

atom.

target

electrons

x-ray

Photoelectric

Effect

A

photon

can

eject

an

electron

from

an

atom.

Photon

is

absorbed

Minimum

energy

needed

for

interaction.

Cross

section

decreases

at

high

energy

e





Z

 

 



h

K

e

Compton

Effect

Photons

scattering

from

atomic

electrons

are

described

by

the

Compton

effect.

Conservation

of

energy

and

momentum





e







’

E

h

mc

h











2

cos

cos

P

c

h

c

h









sin

sin

P

c

h







)

cos

1 (

1

2









mc

h

h

h