General Astronomy
The Nature of Light
Our ideas today on the nature of light
are inductive. That is, from
observations we build a (set of)
model(s) which explain the
observations in a consistant fashion.
We begin by looking at some everyday
observations.
Observation: Reflection
We see reflections everywhere.
From the bathroom mirror in the morning,
to shiny objects to moonlight on the ocean
at night.
The Rule:
The angles in and
out are equal
Note that all
colors reflect at
the same angle.
Observation: Refraction
Refraction is the bending of light when it passes
between two different media. For example,
water and air.
Light will bend toward the normal
when moving from a less to
greater 'index of refraction'
(usually denser material)
Observation: Refraction
Different colors refract at different angles
Refract light through lots of raindrops and you get a rainbow
All colors reflect at the same angle
Observation: Color
• Most of us see color. Our vision
ranges from violet (4000Å) to deep
red (7000Å)
– One Angstrom (Å) is 1x10-8 cm
Corpuscular Model
(Particle Model)
Reflection
Refraction
Particle
Model
Light travels
through a vacuum
Color
Particle Model
Reflection
Particles "bounce"
Angle of incidence = Angle of reflection
Refraction
Particles slow down in denser
material
Observation: Scattering
• Why is the sky blue?
• Why are the clouds white?
• Why are sunsets red?
Observation: Scattering
Blue light is scattered all about
the sky so that where ever we
look we see the blue color
Observation: Scattering
Corpuscular Model
(Particle Model)
Reflection
Scattering
Light travels
through a vacuum
Refraction
Particle
Model
Color
Observation: Polarization
Certain crystals and minerals show curious behavior
under some circumstances. It was noticed that
looking at light reflected off of water through
these crystals brightened and dimmed as the
crystal was rotated in front of the eye
We are having a problem with our model.
How can particles exhibit this kind of behavior?
Observation:Diffraction
Suppose we shine a light through a narrow
opening in a screen, such as sunlight coming
through an opening in a shade. We expect
to see a bright area pretty much in the
same shape as the opening itself.
Top View
Looking at the screen
Diffraction
In many cases, it's more useful
to show this as a plot of
Intensity (brightness) versus
position
Let's close down the slit …
Intensity
position
Single Slit Diffraction
1
As the slit narrows,
instead of the band of
light simply getting
narrower in proportion, it
starts forming bands of
light –
Diffraction Fringes
This is also called a
Diffraction Pattern
The spacing of the fringes
depends on the wavelength
and the slit width
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0.6
0.4
0.2
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0.4
Diffraction
This is hard to explain using the
particle model of light.
If it works for a thin slit, what about a
pinhole?
Circular Diffraction
Passing light through a
small hole, produces
this kind of pattern.
Note that the central
fringe is much, much
greater than the
outer ones.
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0.3
0.2
0.1
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0.2
0.4
10
8
6
4
2
-0.4
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0.2
0.4
Straight Edge Diffraction
Suppose we block half the
light with a straight edge
We expect to see a sharp shadow
Instead we see a diffraction pattern …
1.2
1
0.8
0.6
0.4
0.2
0.5
1
1.5
2
Observation: Interference
What happens if two waves interact with each other?
You can get some pretty complicated ripple patterns if
you tap two fingers on the water surface. We can do
the same with light if we put two slits near each
other:
Interference
Slit separation is 4
times the slit width
The Model
Diffraction
Reflection
Scattering
Light travels
through a vacuum
Refraction
Particle
Model
Polarization
Color
Interference
The Wave Model
• There is another model that might be used
to explain the observations.
• What if light were a wave instead of a
particle?
• First, what do we mean by a wave?
• A wave is a disturbance in a medium.
– Water waves (ripples displacing the water
surface)
– Sound waves (rarifications and compressions in
air)
Wave Properties
The amplitude is how "big" the wave gets
A wavelength is one repetition of the pattern
C = f
Speed of Light = (Wavelength)(Frequency)
The Wave Model
Diffraction
Reflection
Refraction
Scattering
Light travels
through a vacuum
Wave
Model
Polarization
Color
Interference
Reflection, Refraction & Polarization
The Wave Model
• Everything we have observed can be
explained using waves instead of particles –
except one
– If a wave is a disturbance in a medium, what is
the medium in a vacuum?
– It was well known that light could travel
through a vacuum, but it is hard to "ripple".
– The Aether was invented.
• This was an incredibly tenuous medium filling all space
• It supported the high speed of light, but did not put
an appreciable drag on the planets passing through it.
Models
• At this point, the wave model can explain most of
the observations, it can predict the presence of
other new observations.
• Light now appears to be "just another" aspect of
Electromagnetic waves.
Electromagnetic Waves
Status
• We are now close to the turn of the
century (1900 that is).
• The wave theory is becoming more
entrenched and can explain more and more
phenomena.
• The particle model is very much in disfavor.
• Equipment and measurements are getting
more and more accurate.
• Maxwell's Laws predict much of
electromagnetism and electromagnetic
waves --- except for what produces them.
Interlude
And God said,
And there was Light
More Observations
There were four more observations and
experiments which are very important
–
–
–
–
The Doppler Effect
The Photoelectric Effect
Blackbody Radiation
The Michelson-Morley Experiment
The Doppler Effect
• Light will shift color depending on the
speed of the light source.
• Only motion toward or away from you
causes this effect; there is no color
shift for 'sideways' movement.
• Motion toward you shifts the light
toward the blue end of the spectrum
• Motion away from you shifts the light
toward the red end of the spectrum
Consider a stationary source sending out light pulses
This time the light source moves to the right as it pulses
The Doppler Effect
Observer sees redder
light (the wave crests
are farther apart)
A
B
Observer sees bluer
light (the wave crests
are closer together)
Blackbody Radiation
Think about heating an old cast-iron frying pan.
First, you can feel the heat from the pan
Next, you can see a dull red glow
Then it's cherry red
Then orange, yellow, white
Finally it becomes blue-white
The color is an indicator of the temperature
>30,000 °K
6000 °K
4000 °K
3000 °K
Blackbody Radiation
Release of an infinite amount of light at
short wavelengths was known as the
“Ultraviolet Catastrophe”
Max Planck postulated in 1900
that light X-rays, and other waves
(i.e. energy) can only be emitted or
absorbed in discrete amounts
which he called quanta (the plural
of "quantum", the Latin word for
"how much").
The energy quantum is related to
the frequency of the wave by a
new fundamental constant h
The Photoelectric Effect
Under certain circumstances,
light falling on a metal
releases electrons
• The energy of the
electrons is linearly
proportional to the
frequency of the light
• There will be no electrons
if the light is below a
certain frequency
• The amount of electron
flow is proportional to the
intensity of the light.
The Photoelectric Effect
Einstein, using Planck’s idea of a quanta, related
the energy of a quanta – or photon – to it’s
frequency.
The bluer the light, the higher the energy and
the more capable of ‘knocking electrons’ out of
the metal.
The Michelson-Morley Experiment
In the late 1890's, an attempt was made to measure
the motion of the Earth through the 'luminiferous
aether'
An interferometer was designed to detect the
slightest difference in the distance light travels
between two separate paths.
As the Earth moves, one
expects the path
lengths to change
depending on if they
are going with or across
Incoming light
the flow of the aether
Resultant light
There is no measurable change
Which Model ?
Interference
Diffraction
Reflection
Refraction
Photoelectric
Color
Wave
Model
Scattering
Blackbody
Polarization
Doppler
Particle
Model
Light travels through
a vacuum
Which Model ?
• Both!
• It's called the "Wave-Particle Duality"
• It is a model - A view of how it might work.
There is no reason why there cannot be several
equally valid models. We simply choose the one
in which predictions are simplest for a given
observation.
Wave-Particle Duality
Of course, if waves (light) sometimes acts as
if it were a particle (called a photon) then
do particles (electrons, neutrons, etc.)
sometimes act as if they were waves?
YES! Electron microscopes, Electron diffraction are
used to probe the very small structures of nature.
Electrons diffract, interfere and exhibit wave
behavior under the right conditions.