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Not-For-Profit
Public Foundation
Nature of Light
Light is a form of energy.
Energy can be transported by
waves.
Use waves as a tool to understand
light.
WAVE PARAMETERS
• Wavelength (l) is the length from crest to crest or
trough to trough.
• Frequency (f) is the number of waves passing a
point per second.
• Higher Frequency  Shorter Wavelength
• Lower Frequency  Longer Wavelength
• Speed of a wave (v) = Wavelength x Frequency
Light as a WAVE
As a wave, Light can be described as having a
Frequency
Wavelength
Speed
Wave Speed = Wavelength x Frequency
What does a light wave look like?
To answer this, must first know how light
is produced.
• Start with an electron  has an electric
field (example is static cling; or
battery)
• Also a moving electron produces a
magnetic field.
• Accelerate an electron and both
electric and magnetic fields are
produced  called an
electromagnetic wave
The Nature of Light
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Light is an electromagnetic wave
that travels at the speed of light
The speed of light is related to the
wave’s frequency and wavelength
c = lf = 300,000 km/sec
(or 186,000 mi/sec)
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As a wave, light may have any
wavelength (and frequency), but
its speed remains constant.
An electromagnetic wave with a
long wavelength has a low
frequency and one with a short
wavelength has a high frequency.
Visible light is an electromagnetic
wave that has wavelengths
between 400 and 700 nanometers!
(That is, 300 billionths to 700
billionths of a meter!)
• When you have your teeth x-rayed, the dentist is
shining an electromagnetic wave with a wavelength
more than 1,000 times shorter than visible light
• When you listen to the radio, your radio is picking up
electromagnetic waves almost a million times longer
than visible light, or about 0.1 meters.
The range wavelengths (or frequencies)
of electromagnetic waves is called the
electromagnetic spectrum.
Courtesy Lawrence Berkeley National Laboratory
The Spectrometer
Why does each
element have
its own
fingerprint?
Interaction
between light
and atoms…
What are
atoms?
Elementary Particles: Mass and Charge
Particle
Mass
Charge
Proton
1.67 x 10-27
Kg
+1.6 x 1019 Coulomb
Neutron
1.67 x 10-27
Kg
Neutral
Electron
9.11 x 10-31
Kg
-1.6 x 10-19
Coulomb
Build a Model of the Hydrogen Atom
Electron
Proton
Electron
Bohr Model of the Atom and Energy Levels
1. Electrons
Orbit a of
Proton
Bohr Model
the Hydrogen Atom
2. Electrons exist only in discrete orbits and
nowhere in between
3. When electrons absorb a photon of the correct
energy, the electron jumps to a higher energy
level
4. When the electron falls to a lower energy level, a
photon of only one certain energy is emitted
The emitted photons have a discrete energy,
frequency or wavelength that we observe in the
spectrum
Sir William Herschel

More to Light than Meets the eye

Discovery of Infra-red (1800)

(Also discovered Uranus in 1787)
Herschel Experiment
oC
13.4
Herschel Exp
14.6 o C
IR MOVIE
Spectra

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Continuous Spectrum
Thermal Radiation
• (black body radiation)
• Atomic collisions result
excitation of atoms and in
emission of energy at
multiple frequencies.
Emission Spectrum
Excitation of electrons
• High energy level
Return to ground state
produces emission of energy
• Energy produced
proportional to drop
Atomic and Molecular Mechanisms
Interpretation of Spectra
Atomic and Molecular Mechanisms
Jupiter
Infrared
Visible
Radio
Formation of 21cm Radio waves
(1420 MHz)
Visible
Radio at 21 cm
Milky Way Galaxy as Seen in Different Parts of the Spectrum
Multi-spectral Astronomy
w w w . p a r i . e d u
Infrared Movie…