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Unit 5 – Light and Atoms
ASTR 101
Prof. Dave Hanes
The Pessimist
[writing in 1835]
On the subject of stars, all investigations which are not
ultimately reducible to simple visual observations
are…necessarily denied to us… We shall never be able
by any means to study their chemical composition.
August Comte
What Newton Missed
Just as some pianos
have broken keys and
unplayable notes, so
The spectrum of the sun reveals ‘gaps’ (regions
with no light). This is how we know the
composition of the sun (and stars)!!
The Spectrum of the Sun
[a historical drawing: Fraunhofer, 1814]
The Spectrum of a Star
[note the missing colours – the ‘broken piano keys’]
Why Did Newton Miss This?
Lesson: Use a Narrow Slit
Kirchhoff’s First Law
for hot dense bodies
Emission Lamp
low density gas
Kirchhoff’s Second Law
for hot low-density gases
Unique Patterns (‘Fingerprints’)
On Broadway
Kirchhoff’s Third Law
Kirchhoff’s Three Laws
Example: A Stellar Spectrum
The Sun’s Spectrum in Detail
Kirchhoff’s Third Law!
But Where Does It Happen?
In the gases of the Earth’s atmosphere,
just as the starlight reaches us?
In diffuse, spread-out gas between the
stars, filling all space?
In a thin ‘stellar atmosphere’ around the
outskirts of each star?
Many Stars
Simple in Principle:
Collect and Disperse the Light
Complex Details
and Interpretations
(here, light reflected from and emitted by Mars)
Light as Particles:
The Photoelectric Effect
Application: exposure meters in cameras,
automatic doors in elevators
Red Light
Green Light
Blue Light
Consider a Brick Wall
A Wavelength Dependence
Einstein’s Nobel Prize
Light consists not just of waves, but also of discrete lumps
(photons) that act like little ‘bullets’ of energy.
Higher frequency (blue) = more energetic lumps
Lower frequency (red) = less energetic lumps
Individually, they don’t carry much energy.
A 100-Watt light bulb emits almost
300 million trillion photons per second!
Remember the Full Spectrum!
Gamma rays, the highest energy electromagnetic radiation, can
disrupt DNA and cause cancerous mutations.
X-rays can be very penetrating, pass through fleshy tissue
Ultraviolet radiation can damage pigments, tan your skin
Infrared radiation can be felt as glowing warmth
Radio radiation is very low energy. We are awash in it all the
time from radio stations and the like.
The Perplexing Wave-Particle Duality
[a digression for those interested]
So light behaves like a wave but also as a particle. Amazingly, at
the quantum (= small!!) level, so too does all matter, including
electrons (which are so easily visualised as little ‘billiard balls’).
Absorption Lines:
Atoms Provide an Understanding
Racetrack Orbits? No
Quantized Orbits
Quantized Behaviour
To Excite an Atom
1) Heat the gas! Collisions between atoms can ‘bump’ electrons up to higher
levels; as they fall down, we get emission lines.
2) Run an electric current through the gas! This is what happens in neon
3) An orbiting electron can also be raised to a higher energy state by absorbing
a photon of just the right energy. Below, red light has too little energy; blue
light has too much; but green light is just right!
What Then?
Every Atom is Different
- hence forensics!
Beyond Single Atoms
Molecules (both simple and complex) consist of
atoms bound together by the electric attraction of
their electrons and protons. An entire molecule can
rotate or vibrate at various rates.
CO (carbon monoxide) is a simple molecule, shaped
like a baton. If rotating quickly, it can slow down by
emitting a photon.
But only certain changes are possible, like changing
gears on a car. So the emission is quantized – only
photons of certain energies will be observed. Thus
we learn about molecules in space.
Physics History
It is the interaction of atoms with light, and
the science of spectroscopy, that allowed
us to first understand the structure of the
Indeed, these insights led to the modern
theories of quantum mechanics.