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

Lecture 2: Light the Messenger

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:

• Electromagnetic Waves
• Photons (particles of light)

Wave Nature of Light

Can treat light as an Electromagnetic Wave

• Regular fluctuation in the intensity of coupled electric and magnetic fields.
• Wave travels through a vacuum at the speed of light.
• Doesn't need a medium to "wave" in.

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.

Photon:

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

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

Examples:

Radio Waves, Infrared

high energy = high frequency = short wavelength

Examples:

Ultraviolet, X-rays, Gamma Rays

Major Divisions of the Electromagnetic Spectrum

Type of Radiation	Wavelength Range
Gamma Rays	<0.01 nm
X-Rays	0.01-10 nm
Ultraviolet	10-400 nm
Visible Light	400-700 nm
Infrared	700-105 nm (0.1 mm)
Microwaves	0.1-10mm
Radio	>1 cm

The Visible Spectrum

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
Approximate Wavelength	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?

• "Brightness" measures the number of photons per second emitted by the light source

Luminosity (L):

Measure of the total energy output:

• L is measured in Power Units (energy/sec) like Watts
• L is independent of distance

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

• B is measured in Flux Units (energy/sec/area)
• B depends on the distance to the source

Brightness is what we actually measure (an observable property).

The Inverse Square Law of Brightness

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.

The Doppler Effect

• Sound Waves (Siren or Train Horn)
• Light Waves

• relative speed of the source & observer
• direction of motion (together or apart)

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.

Results:

• The cat on the left hears a higher-pitched squeak because the waves have a shorter wavelength (scrunched together by the mouse's motion).

• The cat on the right hears a lower-pitched squeak because the waves have a longer wavelength (spread out by the mouse's motion).

Doppler Effect in Light

A Way to Measure Speeds

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:

• The size of the shift gives the speed of the source
• The color of the shift (Red or Blue) gives the direction of motion (away or towards you).

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 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