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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 2: Light the Messenger

Readings: Chapter 5, sections 5-1, 5-2, & 5-9

Key Ideas

Light is Electromagnetic Radiation
Light as Waves and Photons

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 moves at the speed of light

Can treat light as either:

Wave Nature of Light

Can treat light as an Electromagnetic Wave

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!

Particle Nature of Light

We can also treat light as 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

low energy = low frequency = long wavelength

Radio Waves, Infrared

high energy = high frequency = short wavelength

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

Important Note:

The wavelengths given in the table above are only approximate. Color is a physiological response of our brains to visible light of different energies. Our division of the visible spectrum into named colors is very subjective, and certainly not a matter of even divisions every 50 nm of wavelength! I include approximate wavelengths primarily for purposes of illustration. An oft-cited example of the subjectivity of color names is the color "orange". It did not enter the language until the Middle Ages, when the fruit of the same name reached Europe from the Middle East. Before that, the color would have been called a reddish shade of yellow.

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

Luminosity and Brightness are related through the Inverse Square Law of Brightness:
Inv. 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

Change in observed wavelength of a wave when the source of the waves and observer are moving relative to each other. Examples: 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:

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:

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.

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 the next lecture.
Return to [ Introduction Index | Astronomy 162 Main Page ]
Updated: 2006 January 2
Copyright Richard W. Pogge, All Rights Reserved.