Lecture 7:
Light
Sec 6 (4th Ed)
Sec 5 (3rd Ed)
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Short-wavelength (high-frequency) light
is higher energy radiation
high-energy
photons
E = hν = hc/λ
(h = a fundamental constant, Planck’s constant)
low-energy
photons
What light tells us…
• The color of light emitted by an object can tell us its temperature.
[hotter objects are bluer; colder objects are redder -or-
hotter objects emit more of their light at shorter wavelengths]
What is light?
• Electromagnetic Radiation
–what we call “light” is a type of electromagnetic radiation that we can see (380nm-740nm)
• Light carries energy and can transmit information.
• It moves very quickly, but is not infinitely fast.
Distance from the Earth to the Moon in 1.28 seconds
That’s 384,000 km in 1.28 seconds
Speed of light c ~ 300,000 km/s
What is light?
• Electromagnetic Radiation
–what we call “light” is a type of electromagnetic radiation that we can see (380nm-740nm)
• Light carries energy and can transmit information.
• It moves very quickly, but is not infinitely fast.
Distance from the Earth to the Moon in 1.28 seconds
That’s 384,000 km in 1.28 seconds
Speed of light c ~ 300,000 km/s
Characteristics of waves as they travel
• Waves refract
• A wave bends when it passes from
one medium to another, you see
this when you look through water.
• All waves refract.
Characteristics of waves as they travel
–Waves interfere with each
other
Water wave
interference
Characteristics of waves as they travel
• Waves refract
• Waves interfere with each other
• Waves exhibit Doppler effect
• Waves are compressed in the direction
of travel of their source. They are
stretched relative to you when the source
is moving away from you.
http://archive.ncsa.uiuc.edu
-Sound waves from a siren are compressed towards
the observer. The intervals between waves diminish
=> an increase in frequency or pitch.
-As cop recedes, sound waves are stretched relative
to the observer, causing the siren's pitch to decrease.
-By the change in pitch of the siren, you can
determine if the cop is moving towards or away
from you.
Light Travels Like a Wave
• Light refracts – it bends when it
goes through water or a different
kind of air
Light Travels Like a Wave
• Light waves interfere with each
other – diffraction pattern
Light Travels Like a Wave
• Light refracts
• Light waves interfere with each
other
• Light exhibits Doppler effect.
Astronomers call this “redshift”
Light interacts (hits stuff) like a particle
• The Photo-electric Effect
– Light “hits” like a particle!
–When light shines on a metal, electrons fly
off the metal with an energy that depends
only on the wavelength of the light, not on
how bright the light is.
–This behavior is not expected for waves.
Light interacts (hits stuff) like a particle
• The Photo-electric Effect
– Light “hits” like a particle!
–When light shines on a metal, electrons fly
off the metal with an energy that depends
only on the wavelength of the light, not on
how bright the light is.
–This behavior is not expected for waves.
• 1905 Einstein suggested:
- light deposits energy in “quanta”. Not continuous, but in quantized bits, like a particle.
• These light particles are called “photons”.
• Energy of the photons depends on wavelength.
What is Light?
• Light travels like a wave
–It Refracts
–Shows Interference
–Exhibits the Doppler effect
• Light “hits” like a particle!
–Light deposits energy in “quanta”. Not
continuous, but in quantized bits, like a
particle.
–These light particles are called “photons”.
What is Light?
• Light travels like a wave
–It Refracts
–Shows Interference
–Exhibits the Doppler effect
?
• Light “hits” like a particle!
• This particle-wave
“duality” is intimately
linked to the theory of
Quantum Mechanics,
developed by physicists in
the early 20th century.
According to Quantum Mechanics
everything has particle/wave duality.
It only “shows up” on very small length
scales, e.g., atoms, electrons, light, etc.
What light tells us…
• The color of light emitted by an object can tell us its temperature.
[hotter objects are bluer; colder objects are redder -or-
hotter objects emit more of their light at shorter wavelengths]
• Light can tell us about the velocity of an emitting object. [via the doppler shift]
The Doppler Effect
Stationary
source of waves
The Doppler Effect
Moving source
of waves
Person over here
hears a lower pitch
Person over here
hears a higher pitch
Doppler Effect: Summary
If the source of waves is moving toward you, you’ll see waves
of shorter wavelength.
For light waves, this is called a blueshift.
If the source of waves is moving away from you, you’ll see
waves of longer wavelength.
For light waves, this is called a redshift.
By measuring the amount of blueshift or redshift, we can
determine the object’s velocity toward or away from us.
Note: the speed of the waves is not affected!
Doppler shift of light
The Doppler effect depends only on the object’s motion along
a direction toward or away from the observer
Doppler shift of light
The Doppler effect depends only on the object’s motion along
a direction toward or away from the observer
If star isn’t moving relative to
the observer, then the
observed spectrum will not
be Doppler-shifted
Doppler shift of light
The Doppler effect depends only on the object’s motion along
a direction toward or away from the observer
Star moving this way:
Observer sees redshifted
spectrum
Doppler shift of light
The Doppler effect depends only on the object’s motion along
a direction toward or away from the observer
Star moving this way:
Observer receives light that
is not Doppler-shifted
Doppler shift of light
Important note: Doppler effect depends only on the object’s
motion along a direction toward or away from the observer
Star moving this way:
The Doppler shift only
depends on the
component of the star’s
motion toward or away
from the observer
Doppler shift of light
Important note: Doppler effect depends only on the object’s
motion along a direction toward or away from the observer
Star moving this way:
The Doppler shift only
depends on the
component of the star’s
motion toward or away
from the observer
Question
Star at rest
relative to observer:
No shift
Is this star moving
toward the observer,
or away?
Wavelength (nm)
Star at rest
relative to observer:
No shift
Wavelength (nm)
Star at rest
relative to observer:
No shift
Star moving away
from observer:
Spectrum is redshifted
Wavelength (nm)
Star at rest
relative to observer:
No shift
Star moving away
from observer:
Spectrum is redshifted
Star moving
toward observer:
Spectrum is blueshifted
Wavelength (nm)
Doppler shift of light
v
∆λ
=
λ0
c
Note: this equation is valid when
the star’s velocity is very small
compared with the speed of light!
Doppler shift of light
v
∆λ
=
λ0
c
Note: this equation is valid when
the star’s velocity is very small
compared with the speed of light!
λ0 = the “rest wavelength” of the photon
λobs = the observed wavelength, measured by the observer
Δλ = λobs - λ0 = the shift in wavelength
v = the velocity of the source relative to the observer
c = speed of light
If v is negative, the source is moving toward us.
If v is positive, the source is moving away from us.
Energy
v
∆λ
=
λ0
c
Wavelength →
v
∆λ
=
λ0
c
Energy
Star at rest
Wavelength →
800.0 nm
v
∆λ
=
λ0
c
Star at rest
Energy
Star moving away
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
Energy
Star moving away
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
λobs = 800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
Δλ/λ0 = 0.8/800 = 0.001
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
Δλ/λ0 = 0.8/800 = 0.001
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
c = 3×108 m/sec
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
Δλ/λ0 = 0.8/800 = 0.001
How fast is this
star moving
away from us?
Wavelength →
800.0 nm
800.8 nm
c = 3×108 m/sec
v = 0.001 × c
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
Δλ/λ0 = 0.8/800 = 0.001
How fast is this
star moving
away from us?
c = 3×108 m/sec
v = 0.001 × c
So, v = 3×105 m/sec
Wavelength →
800.0 nm
800.8 nm
v
∆λ
=
λ0
c
Star at rest
λ0 = 800.0 nm
Energy
Star moving away
λobs = 800.8 nm
Δλ = λobs - λ0 = 0.8 nm
Δλ/λ0 = 0.8/800 = 0.001
How fast is this
star moving
away from us?
c = 3×108 m/sec
v = 0.001 × c
So, v = 3×105 m/sec
Wavelength →
800.0 nm
800.8 nm
or 300 km/sec
Doppler shifts from rotating objects
If an object is rotating, then light from different parts of the
object will have different doppler shifts
Blueshifted
emission
from this
side of disk
Redshifted
emission
from this
side of disk
What light tells us…
• The color of light emitted by an object can tell us its temperature.
[hotter objects are bluer; colder objects are redder -or-
hotter objects emit more of their light at shorter wavelengths]
• Light can tell us about the velocity of an emitting object. [via the doppler shift]
• Light can tell us about the composition of objects.
[different atoms/molecules emit (or absorb) light at specific wavelengths]