1

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.

2

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.

3

Sources of Light vs. Reflected Light• The Sun is the primary light source illuminating Solar System

objects.

4

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).

6

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

E=hν=hcλ

7

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

E=hν=hcλ

8

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).

9

Spectra• Light can be sorted and binned by wavelength. The resulting

spectrum can be projected on a screen or plotted on a graph.

10

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

11

The Solar Spectrum

12

Continuous Spectra: Thermal Radiation• Any hot object glows

The hotter the object the brighter and bluer the glow

13

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.

14

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.

15

Continuous Spectra: Thermal Radiation• Any hot object glows

The hotter the object the brighter and bluer the glow

16

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.

17

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!

18

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.

19

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

20

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

21

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

25

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.

26

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).

λshifted− λrestλrest

=Δλλrest

=vc

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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.

λshifted− λrestλrest

=Δλλrest

=vc

28

The Doppler Shift• 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!

λshifted− λrestλrest

=Δλλrest

=vc