Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 20: Light

Key Ideas:

Speed of Light

Olaus Romer noticed that the timing of the eclipses of Jupiter's moons depended on the Earth-Jupiter distance: Eclipses occur 16.6 minutes later when Jupiter is at conjunction compared to opposition; 2 AU is therefore equal to the distance light travels in 16.6 minutes

Fizeau-Foucault Method

In 1850 the French physicists Fizeau and Foucault measured the speed of light in their laboratory:: c = 299,792.458 km/s
In their experiment, the deflection angle increases if:: The mirror rotates faster; There is a larger path length through the experiment

Waves

A "Wave" is any periodic change in the properties of a medium that travels through it and carries energy: "periodic" = with a regularly repeating pattern
Examples:: Water Waves: periodic changes in the height of the water traveling across the surface; Sound Waves: periodic changes in air pressure (compression waves) traveling though the air

Measuring Waves

Examples of Waves

Ocean Waves:: l = 100m, f = 0.1/second; wave speed: c = 10 m/s (36 km/hr); Speed depends on water depth, salinity, etc.
Sound Waves: (A 440 cycles per second): l = 0.73 m, f = 440/second or 440 Hertz (Hz); wave speed: c = 320 m/s (1150 km/hr); For sound, "frequency" = "pitch"; Sound waves are pressure changes

Light as a Wave

Can treat light as an Electromagnetic Wave: Periodic changes in the strengths of electric and magnetic fields; Travels through a vacuum at the speed of light; Doesn't need a medium to "wave" in
Speed of light is a constant for all light waves: c = 300,000 km/sec; Independent of wavelength or frequency

Visible Light Waves

Light as a Particle

Photoelectric Effect

Demonstration of the particle nature of light: Light hitting a piece of metal (for example, Cesium) kicks out electrons.
Low-freequency light (e.g. red): No electrons kicked out, no matter how bright
High-frequency light (e.g. blue): Number of electrons kicked out is proportional to the brightness of the light; Electron energies are proportional to the frequency
Photons hitting the metal will knock out single electrons only if they have enough energy to break the electrons free of the metal.
Demonstration of this won Einstein the Nobel Prize

Electromagnetic Spectrum

Sequence of photon energies from low to high is called the Electromagnetic Spectrum
Low energy = low frequency = long wavelength: Examples are radio waves, microwaves, infrared
High energy = high frequency = short wavelength: Examples are ultraviolet, X-rays, Gamma Rays

How "Bright" is a Light Source?

We need to quantify how bright an object is
Wave picture of light:: Brightness is the amplitude of the wave (height of the wave crests)
Particle (photon) picture of light:: Brightness is the number of photons per second from the light source
The photon picture is the more useful one

Luminosity

Luminosity is the total energy emitted by an object: Measured in Power Units:; Energy per second emitted (e.g. Watts); Independent of Distance
Measures the total energy output of an object (e.g. the Sun)

Apparent Brightness

Measures how bright an object appears to be to a distant observer: What we actually measure directly (observable)
Measured in Flux Units:: Energy per second per area from the source
Depends on the distance to the object

Inverse Square Law of Brightness

The Apparent Brightness of a source is inversely proportional to the square of its distance: 2 times closer = 4 times brighter; 2 times farther = 4 times fainter

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.