Transcript File

Light as a
Wave
SPH4U
Young Star Cluster NGC 7129
What is Light?
All charged particles have an
electric field. When they move,
they change the electric field
(and create a magnetic field).
These field distortions propagate
through space as an
electromagnetic wave, aka light.
Light is a charged particle saying,
“Hey, I just moved.”
Electromagnetic Waves
The electric and magnetic field distortions are
perpendicular to each other and to the
direction of propagation:
Wavelength
The wavelength is determined by the scale of
the charged particle’s displacement:
A displacement within an
atomic nucleus (10-12 m)
will create light with a
wavelength of 10-12 m.
A displacement within a
radio antenna (1 m) will
create light with a
wavelength of 1 m.
Wavelength
The wavelength is determined by the scale of
the charged particle’s displacement:
A displacement within an
atomic nucleus (10-12 m)
will create light with a
wavelength of 10-12 m.
gamma
rays
A displacement within a
radio antenna (1 m) will
create light with a
wavelength of 1 m.
radio
waves
Light Production by Wavelength
•
•
•
•
•
•
Big Metal Antennae: Radio
Vibrating Molecules: Infrared
Excited Electrons: Visible Light
Chemical Bonds: UV
Ionized Electrons: X-rays
Atomic Nuclei: Gamma-Rays
(Note that there is more energy involved as you go to
shorter wavelengths.)
The Electromagnetic Spectrum
The Electromagnetic Spectrum
Visible light is a very small part of the spectrum:
between 700 nm (red) and 400 nm (violet).
Human Perception of Light
The colour detectors on
our retinas (cones)
are sensitive to red,
green, and blue.
Light Speed
Electromagnetic waves travel at:
vlight  c  3.0 10
8 m
s
in a vacuum.
Light and
Radiation
Astronomical distances are measured in light
travel time:
• Planets/Sun: Light minutes away
• Nearest Stars: Light years away
• Nearest Galaxies: Millions of Light years
• The Edge of the Observable Universe:
14 Billion Light years
(light from further away hasn’t had time to reach us yet)
Frequency
Frequency is how many crests arrive each
second:
# of waves
 Hz
s
The Wave Equation
d
v
t
 1 
v  d  
 t 
c  f
Example
What is the frequency of a light wave with a
wavelength of 420 nm?
Example
What is the frequency of a light wave with a
wavelength of 420 nm?
  420 109 m
c  3.0 10
f ?
8 m
s
Example
What is the frequency of a light wave with a
wavelength of 420 nm?
  420 10 m
9
c  3.0 10
f ?
8 m
s
c  f  f 
c

3.0  10
f 
420  10 m
14
f  7.1 10 Hz
8 m
s
9
Blackbody
Radiation
Objects emit radiation
(lose energy from
particle motion) at
wavelengths related
to their temperatures:
Blackbody curve
Blackbody
Radiation
Wien’s Law
MAX
3
2.9 10 m

T in Kelvin
What is your peak wavelength?
What is your peak wavelength?
Body temperature: 310 K
What is your peak wavelength?
Body temperature: 310 K
MAX
3
2.9 10 m

 9400 nm
310
too long to be
visible: peak is in
the infrared
The light seen here is
reflected visible light.
This is the emitted
infrared.
T (Fahrenheit)
This is the emitted
infrared.
Cold-blooded
Emitted Energy
This light is emitted in all directions:
L
F
Area
L (Luminosity): total light energy/time (Watts)
F (Flux): light energy/time/unit area (Watts per m2)
Emitted Energy
This light is emitted in all directions:
L
F
2
4 r
L (Luminosity): total light energy/time (Watts)
F (Flux): light energy/time/unit area (Watts per m2)
Doppler Shift
Light also exhibits
other wave
behaviours,
e.g., the Doppler
Effect
Doppler Shift
• For an unmoving
source of light, the
waves arrive with
the same spacing in
all directions.
• The wavelength is
simply the space
between each wave.
Doppler Shift
• For a moving source
of light the waves in
front bunch up – the
wavelength gets
shorter!
• Blue light is shorter
wavelength: we call
this a blue shift.
Doppler Shift
• For a moving source
of light the waves
behind spread out –
the wavelength gets
longer!
• Red light is longer
wavelength: we call
this a red shift.
Doppler Shift
Red-shifted
absorption lines in
the spectrum of a
galaxy moving
away from ours.
(Non-relativistic) Doppler Shift
OBSERVED
f TRUE
v

 1
TRUE
f OBSERVED
c
•
•
•
•
 wavelength of signal
f frequency of signal
v velocity of recession (away)
c speed of signal
Example
A source’s blue hydrogen line is shifted
from 486.1 nm to 537.4 nm. What is
the speed of the source relative to us?
Example
A source’s blue hydrogen line is shifted
from 486.1 nm to 537.4 nm. What is
the speed of the source relative to us?
obs  537.4 nm
true  486.1 nm
c  3.0  108
v?
m
s
Example
A source’s blue hydrogen line is shifted
from 486.1 nm to 537.4 nm. What is
the speed of the source relative to us?
obs  537.4 nm
true  486.1 nm
c  3.0  108
v?
m
s
 obs 
obs
v
 1   v  c
 1
true
c
 true 

v  3.0  10
8 m
s
v  3.2  107
m
s
 537.4 nm 

 1
 486.1 nm 

Example
A source’s blue hydrogen line is shifted
from 486.1 nm to 537.4 nm. What is
the speed of the source relative to us?
obs  537.4 nm
true  486.1 nm
c  3.0  108
v?
m
s
 obs 
obs
v
 1   v  c
 1
true
c
 true 

v  3.0  10
8 m
s
v  3.2  107
m
s
 537.4 nm 

 1
 486.1 nm 

[away from us]
More Practice
“Light as a Wave Homework Questions”