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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 22: Light the Messenger

Key Ideas:

Light is Electromagnetic Radiation
Light as Waves or Photons
Light carries energy at the speed of light

The Electromagnetic Spectrum
Sequence of photon energies

Luminosity vs. Apparent Brightness
Inverse Square Law of Brightness

Doppler Effect
Due to relative motion between source & observer
Way to measure speeds at a distance

Electromagnetic Radiation

Light is Electromagnetic Radiation, a self-propagating Electromagnetic disturbance that carries energy at the speed of light

Two complementary ways to describe light

Wave Nature of Light

Light can be described as Electromagnetic Waves The speed of light (c) is the same for all light waves:

c = 299,792.458 km/sec

This speed is independent of the wavelength or frequency of the light wave! [22.1]

Particle Nature of Light

We can also describe light as a stream of particles or Photons.


Massless particles that carry energy at the speed of light.

Photons are characterized by their Energy, which is proportional to their Frequency, f.

Photon Energy:

E = hf
where: f = frequency of the light, and h = Planck's Constant

In words:

Higher Frequencies mean Higher Energies

Note: Energy, E, is a "particle property", whereas frequency, f, is a "wave property". They are related through Planck's Constant, h. Planck's constant is one of the "Fundamental Constants" of Nature (another one we've met in this lecture is the speed of light, c). One can think of Planck's constant as serving as the "link" between the wave and particle natures of light, and indeed h is the fundamental constant in "quantum mechanics", the modern view of matter and radiation as entities ("quanta") that have the properties of both waves and particles.

The Electromagnetic Spectrum

The sequence of photon energies running from low energy to high energy is called the Electromagnetic Spectrum

Photon Energy

low energy = low frequency = long wavelength
Examples of low-energy photons: Radio Waves, Infrared

high energy = high frequency = short wavelength
Examples of high-energy photons: Ultraviolet, X-rays, Gamma Rays

Major Divisions of the Electromagnetic Spectrum

Type of RadiationWavelength Range
Gamma Rays<0.01 nm
X-Rays0.01-10 nm
Ultraviolet10-400 nm
Visible Light400-700 nm
Infrared700-105 nm (0.1 mm)
Radio>1 cm

The Visible Spectrum

This is all forms of light we can see with our eyes.
Wavelengths: 400 - 700 nanometers (nm)
Frequencies: 7.5x1014 - 4.3x1014 waves/second

We sense visible light of different energies as different colors. The basic colors of the visible spectrum are defined roughly as follows, in order of increasing photon energy:

Color Name Red Orange Yellow Green Blue Indigo Violet
700nm 650nm 600nm 550nm 500nm 450nm 400nm

You can remember the order of these colors from lowest to highest energy using the famous mnemonic: ROY G. BIV

[22.2 - note about colors and wavelengths]

How "Bright" is a Light Source?

We need to quantify how bright a light source is. The most convenient way is using the photon picture for light: There are two ways to quantify this:
Luminosity (L):
Measure of the total energy output:
Luminosity is an intrinsic property of the light source.

Apparent Brightness (B):
Measures how bright an object appears to be as seen from a distance
Brightness is what we actually measure (an observable property).

The Inverse Square Law of Brightness

As light travels away from its source, the photons get spread out over increasingly greater areas. The more the light is spread out, the fainter it appears in any given patch of area (for example, the small patch of area covered by the opening in your eye):

Inverse Square Law Illustration

This means that the Apparent Brightness, B of a light source with intrinsic Luminosity L will get fainter as the square of our distance from the source:

Inverse Square Law of Brightness
In words:
Apparent Brightness is inversely proportional to the square of the distance to the source
If the source is 2x closer, it appears 4x brighter.
If the source is 2x farther away, it appears 4x fainter.
This law is extremely important to us in astronomy.

The Doppler Effect

The observed wavelength of a wave will change when the source of the waves and the observer are moving either towards or away from each other.


The amount of the shift and its sign depends on

Doppler Effect in Sound

Two cats are sitting between a windup mouse toy that emits an electronic squeak. The mouse is moving towards the left towards the first cat and away from the second:
Doppler Effect in Sound
The mouse emitted a squeak when it was at the location of each of the green dots. The sound wave ("squeak!") moves outward spherically from each point of emission. Because the mouse is moving, the sound waves have different emission centers. Those waves ahead of its motion (towards the left) are are scrunched together, while those behind are spread out.


Doppler Effect in Light

The Doppler Effect in light works the same way as it does for sound:

A Way to Measure Speeds

Observe the wavelength lobs of a light source with a known emitted wavelength lem.

The difference between the observed and emitted wavelengths is directly proportional to the speed of the source towards or away from you (v), given by the Doppler Formula:

Doppler Effect Formula
Here c is the speed of light.

The Doppler Effect in Practice

The Doppler Effect in light is used by astronomers to measure the speeds of objects moving towards or away from the Earth.

But, we also use the Doppler Effect in light in everyday settings. Some examples:

Traffic Radar Guns:
Radar gun bounces a pulse of microwaves (or infrared laser light) of a known wavelength off a car or truck, measure the wavelength reflected back. The Doppler shift gives the vehicle's speed. [22.3]

Doppler Weather Radar:
Similar principle, bounce microwave radar signals of known wavelength off of clouds, measure the wavelength reflected back. The Doppler shift and its sign (blue or red) gives the speed and direction of the clouds. The strength of the returned signal also gives the amount of rain or snow falling.

Light the Messenger

The properties of light give us a way to bridge the vast distances between us and astronomical objects.

The light "encodes" important information about the source of the light

  1. Luminosity measures how much energy the object emits in the form of light.
  2. Doppler Shift tells us how fast it is moving towards or away from us
To learn more from light, however, we need to understand how light and matter interact, which is the subject of next few lectures.
Return to [ Unit 4 Index | Astronomy 161 Main Page ]
Updated: 2007 October 19
Copyright Richard W. Pogge, All Rights Reserved.