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Electromagnetic Radiation (Light)
•
•
A source of light produces packets of energy called “photons”
Each packet has a well defined wavelength (which we perceive as
color at visible wavelengths), the separation between wavecrests
of the electromagnetic wave.
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Sources of Light vs. Reflected Light
•
The vast majority of the things we see are made visible by
reflected light originating from one or more sources of light.
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Sources of Light vs. Reflected Light
•
The Sun is the primary light source illuminating Solar System
objects.
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Sources of Light vs. Reflected Light
•
The Moon, planets, asteroids, etc. “shine” by reflected sunlight.
5
Electromagnetic Radiation (Light)
•
•
A source of light produces packets of energy called “photons”
Each packet has a well defined wavelength (which we perceive as
color at visible wavelengths).
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Wavelength
•
Wavelength alone distinguishes types of light

At visible wavelengths – short wavelengths are blue; long are red

Wavelength, color, and energy of a photon are all the same thing
λ∗ν=c
hc
E=hν=
λ
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Wavelength
•
Wavelength alone distinguishes types of light

At visible wavelengths – short wavelengths are blue; long are red

Wavelength, color, and energy of a photon are all the same thing
• Short wavelength photons (the “bluer” ones) carry more energy
than long wavelength photons (the “redder” ones).
• Start thinking, now, about “blue” and “red” being directions in the
spectrum rather than absolutes
– “toward the blue...” = “toward shorter wavelengths”
λ∗ν=c
hc
E=hν=
λ
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The Electromagnetic Spectrum
•
Wavelength alone distinguishes types of light



Visible light covers a tiny range of possible wavelengths
We have used technology to make other wavelengths “visible”
defining, in the process new regions of the spectrum.
Radio, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray are all
forms of light of different wavelength (here from long wavelengths to
short).
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Spectra
•
Light can be sorted and binned by wavelength. The resulting
spectrum can be projected on a screen or plotted on a graph.
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Two Fundamental Types of Spectra
•
•
Spectra can be from one of two classes

Continuous – a smoothly varying distribution of all colors

Discrete – emission (or absorption) at precise wavelengths
Often a spectrum is a combination of both
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The Solar Spectrum
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Continuous Spectra: Thermal Radiation
•
Any hot object glows

The hotter the object the brighter and bluer the glow
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The Nature of Temperature
•
Temperature is a measure of the energy of motion of particles in a
gas or in a solid.

In a gas the particles (atoms or molecules) are independently flying
about colliding with one another or with the walls of the chamber.



At high temperature the particles move quickly. At low temperatures
they are sluggish.
In a solid the particles are vibrating in place.
The lowest possible temperature is the point at which all thermal
energy has been removed – absolute zero.
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The Nature of Temperature
•
Temperature is a measure of the energy of motion of particles in a
gas or in a solid.

In a gas the particles (atoms or molecules) are independently flying
about colliding with one another or with the walls of the chamber.



At high temperature the particles move quickly. At low temperatures
they are sluggish.
In a solid the particles are vibrating in place.
The lowest possible temperature is the point at which all thermal
energy has been removed – absolute zero.
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Continuous Spectra: Thermal Radiation
•
Any hot object glows

The hotter the object the brighter and bluer the glow
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Continuous Spectra: Thermal Radiation
•
Dense spheres of gas (stars) are good approximations to
blackbodies as well.

The hot stars below are blue. Cooler ones are yellow and red.
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Continuous Spectra: Thermal Radiation
•



The equations below quantitatively summarize the light-emitting
properties of solid objects.
The hotter the object the “bluer” the glow.
The Sun (6000K) peaks in the middle of
the visible spectrum
(0.5 micrometers /
500 nanometers)
Room temperature objects (300K) peak
deep in the infrared (10 um).



The hotter the object the “brighter” the glow.
The energy emitted from each square
centimeter of the surface of a hot object
increases as the fourth power of the
temperature.
Double the temperature and the emission
goes up 16 times!
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Sunspots and Thermal Radiation
•
Sunspots are relatively cooler regions of the Sun's 6000K surface.

Being only about 1000K cooler than their surroundings, they do glow
brightly, but due to the strong, T4, dependence of a hot solid object's
brightness on its temperature they appear dark.
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Spectral Line Emission/Absorption
•
Individual atoms produce/absorb light only at precise discrete
wavelengths/colors (or specifically at certain exact energies).
http://jersey.uoregon.edu/vlab/elements/Elements.html
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Spectral Line Emission/Absorption
•
This property arises from the discrete nature of electronic “orbits” in atoms.
•
Electrons can only be in configurations that have a specific energy.


Jumping between these configurations (higher to lower energy) emits light.
A photon of exactly the right energy can kick an electron from a lower to higher
energy.
http://jersey.uoregon.edu/vlab/elements/Elements.html
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Spectral Line Emission/Absorption
•
This property arises from the discrete nature of electronic “orbits” in atoms.
•
Electrons can only be in configurations that have a specific energy.


Jumping between these configurations (higher to lower energy) emits light.
Conversely, a photon of exactly the right energy can kick an electron from a lower to
higher energy.
http://jersey.uoregon.edu/vlab/elements/Elements.html
22
Spectral Line Emission/Absorption
•
This property arises from the discrete nature of electronic “orbits” in atoms.
•
Electrons can only be in configurations that have a specific energy.


Jumping between these configurations (higher to lower energy) emits light.
A photon of exactly the right energy can kick an electron from a lower to higher
energy.
23
Spectral Line Emission/Absorption
•
This property arises from the discrete nature of electronic “orbits” in atoms.
•
Electrons can only be in configurations that have a specific energy.


Jumping between these configurations (higher to lower energy) emits light.
A photon of exactly the right energy can kick an electron from a lower to higher
energy.
http://jersey.uoregon.edu/vlab/elements/Elements.html
24
Spectral Line Emission/Absorption
•
Spectral lines can reveal the elemental content of a planet or star's atmosphere.
•
Line intensity reveals both the quantity of the element as well as the temperature.
http://jersey.uoregon.edu/vlab/elements/Elements.html
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Spectral Line Emission/Absorption
•
Spectral line absorption arises when light from a continuous source passes
through a cold gas.

The gas atoms selectively remove (actually scatter) specific colors/energies.
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The Doppler Shift
•
The observed wavelength of a spectral line depends on the velocity of the
source toward or away from the observer.
•
The amount of the shift is proportional to the object's velocity relative to the
speed of light (so typically the shift is tiny but measurable).
λ shifted − λ rest
λ rest
Δλ v
=
=
λrest c
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The Doppler Shift
•
Objects approaching an observer have wavelengths artificially shifted toward
shorter wavelengths – a blueshift.


Objects moving away toward longer wavelengths – a redshift
Note that these are directions in the electromagnetic spectrum, not
absolute colors.
λ shifted − λ rest
λ rest
Δλ v
=
=
λrest c
The Doppler Shift
•
λ shifted − λ rest
λ rest
Δλ v
=
=
λrest c
Using the Doppler Shift we can measure the subtle motions (towards or away
from us) of stars, galaxies and interstellar gas without ever seeing actual
movement!
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