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Key Concepts: Lecture 19: Light
Light: wave-like behavior
Light: particle-like behavior
Light: Interaction with matter - Kirchoff’s Laws
The Wave Nature of Electro-Magnetic Radiation
• Visible light is just one
form of electromagnetic radiation
• Light often behaves like
a wave
–Diffraction: waves
bend around obstacles
–Interference: waves
add their amplitudes
Recall: Wave Nature of Electro-Magnetic Radiation
Recall: The Importance of Light
• We can not hope to visit objects outside the solar
system in our lifetime.
• The easiest way we can learn about distant objects is
by studying the light they emit.
• Using information in their light, we can measure
compositions, temperatures, distances, masses and
ages of astronomical objects.
• As light travels at a finite speed, by looking further
and further away we look back further and further
into the past.
All EM radiation is composed of two waves that move together
An electric field and a magnetic field
The fields are at 90 degrees to each other
It needs no medium to be transmitted through, i.e. it can travel through a vacuum
Energy is carried by the wave
Recall: Properties of Waves
• A wave is described by four properties
– The wavelength (λ) - units of length
–The amplitude of the wave
–The speed of the wave (c) - units of speed length/time
–The frequency of the wave (f) - units of 1/time
• Three of these properties are interrelated: c = f x λ
•EM spectrum
extends from
radio to gamma
•The Earth’s
atmosphere only
transmits certain
Light Also Behaves Like A Particle!
• When light interacts with matter, it
often behaves like a particle
• A particle of light is called a Photon
• First suggested by Max Planck (1899)
– The energy of a particle of light (photon)
depends only on its frequency
– h is a constant known as Planck’s constant
• Photoelectric effect - Einstein (1905)
– Certain metals emit electrons when exposed to light.
Basis of exposure meters, night vision goggles etc...
– This only happens when a high enough frequency (i.e.
short enough wavelength) light is used, because a
certain amount of energy is needed to free each
electron from the metal and that energy must come
from a single photon of high enough energy. Lots of
low energy photons don’t work.
Wave-Particle Duality
• Light sometimes behaves like a
particle, sometimes like a wave.
• Traditional “particles”, e.g. electrons,
protons, bullets, when moving also
sometimes behave like waves, but
the effects are only noticeable on
scales of the wavelength, which are
typically minuscule and too small to
see in everyday life.
• Quantum mechanics describes the
behavior of these “wave-particles”.
All objects have this behavior, but in
everyday life we don’t notice it.
The Interaction of Light with Matter
Wave-Particle Duality Video
• The possible energies which
an electron can have are
quantized: only certain
particular energies are
• Electrons and photons can
exchange energy
– An atom can absorb a
photon by moving an
electron to a higher energy
– An atom can emit a photon
by an electron moving to a
lower energy state
– The change in the energy
of the electron must
exactly equal the energy of
the photon
The Interaction of Light with Matter
• Matter is composed of atoms
– Atoms have nuclei composed of
Protons (+ve electric charge)
Neutrons (neutral)
– The nuclei are surrounded by a cloud of
Electrons (-ve electric charge)
– The number of protons determines what
element it is. To be electrically neutral the
number of electrons must equal the number
of protons
• Light typically interacts with matter by
interacting with the electrons around atoms
The Stair Case Analogy
• You can think of the levels of an
atom as being like a flight of stairs
• To step up to a higher level
requires enough energy to lift you
one or more stairs
• When you come back down the
stairs you give up the energy you
got by going up the stairs
Finger Printing the Elements
• Each type of atom (i.e. different elements like Hydrogen,
Helium, Carbon, Oxygen, etc) has a unique configuration
of electrons
• Each type of atom has a unique set of possible energy
• Each type of atom emits and absorbs photons with a
unique pattern of energies
Kirchhoff’s Laws
• Gustav Kirchoff derived three laws to
explain how matter and light interact
• He noted that three types of spectra
arise under different conditions
– Continuous spectra
– Emission line spectra
– Absorption line spectra
The Generation of Light
Spectra of Atoms
• All dense matter with a temperature above absolute zero emits
light because of vibrating electric charges (electrons)
• The spectrum is the amount (intensity) of light at different
frequencies (or wavelengths)
• The peak of the spectrum depends
on the temperature:
hotter temperature leads to peak at
higher frequencies (shorter wavelengths)
• This is called
Blackbody Radiation
which is an example of a
continuous spectrum
Wien’s Law
• Wien’s law tells you at what
wavelength (or frequency) the
blackbody spectrum peaks (has
the maximum value)
– Hotter objects emit light that peaks
at shorter wavelengths (higher
frequencies & higher energy
– Colder objects emit a spectrum
that peaks at longer wavelengths
(lower frequencies & lower energy
λmax = 0.29cm
T (K)
Use of Wien’s Law
We can use Wien’s law to find the temperatures of distant
bodies we can never visit, by measuring the wavelength
(or the frequency) at which the spectrum peaks.
Kirchhoff’s 1st Law
• A hot and opaque
medium emits a
continuous spectrum,
because the atoms are
very close to each other
which smears out the
energy levels.
• This is the blackbody
Kirchhoff’s 2nd Law
• A hot, transparent, low-density
gas produces a spectrum of
bright emission lines. The
number and colors of the lines
depend on which elements are
present. Because the atoms are
quite far apart the energies of
the photons are not smeared
• The emission lines are due to
electrons moving from higher
to lower energy states
Kirchhoff’s 3rd Law
• If a continuous spectrum
passes through a transparent
gas at a lower temperature,
the cooler gas will cause the
appearance of dark
absorption lines, whose colors
and number will depend on
the elements present in the
• This is because particular
energies are absorbed by the
cool gas (electrons are raised
to higher energy levels). Then
when the electrons fall down
and new photons emitted,
these photons can go in any
direction: most do not go
through the slit.
An absorption spectrum
is produced when light
passes through a cold gas
Summary of Kirchoff’s Laws