Waves and Properties of Light

Introduction Information from the Skies Waves in What? The Wave Nature of Radiation The Electromagnetic Spectrum Thermal Radiation The Kelvin Temperature Scale More about the Radiation Laws The Doppler Effect

Information from the Skies Electromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields Example: Light

Wave Properties Wave motion: transmits energy without the physical transport of material

Wave Properties Example: water wave Water just moves up and down Wave travels and can transmit energy

Wave Properties Frequency: number of wave crests that pass a given point per second Period: time between passage of successive crests Relationship: Frequency = 1 / Period

Velocity = Wavelength / Period Wave Properties Wavelength: distance between successive crests Velocity: speed at which crests move Relationship: Velocity = Wavelength / Period

Light Visible spectrum:

Waves in What? Water waves, sound waves, and so on, travel in a medium (water, air, …) Electromagnetic waves need no medium Created by accelerating charged particles:

Waves in What? Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa

Waves in What? What is the wave speed of electromagnetic waves? c = 3.0 × 108 m/s This speed is very large, but still finite; it can take light millions or even billions of years to traverse astronomical distances

Waves in What? The wave nature of radiation: radiation diffracts, which is purely a wave phenomenon

The Electromagnetic Spectrum No limit on wavelengths Different ranges have different names

The Ultraviolet Catastrophe Recall that shorter waves correspond to higher energy. As objects get hotter and hotter they should emit more and more energy at higher and higher frequencies or energies. This was not observed in the spectrum. Instead a sharp drop in intensity is observed!!

Thermal Radiation Blackbody Spectrum: radiation emitted by an object depending only on its temperature

Thermal Radiation Temperature is a measure of the velocity of high speed molecules impacting the thermometer bulb. Kelvin Temperature scale: All thermal motion ceases at 0 K Water freezes at 273 K and boils at 373 K

Thermal Radiation Radiation Laws Wien’s Law Peak wavelength is inversely proportional to temperature, lp a 1/T. or Peak frequency is proportional to the temperature, fp a T.

Thermal Radiation Radiation Laws Stefan-Boltzmann Law 2. Total energy emitted is proportional to fourth power of temperature (note intensity of curves) E a T4 E is the energy emitted per unit of area.

NOS and Scientific Laws Laws are descriptions of phenomena such as the two radiation laws previously discussed. These are simply observed to happen in a particular way such as E a T4. Theories are models that explain why phenomena occur, or why laws are obeyed. So E a T4 is the Stefen-Bolzmann Law, which is explained by Kinetic Theory and Quantum Mechanics that relates the energy of moving particles, like molecules, to the temperature of an object.

Star Color and Temperature We observe stellar colors and get their temperatures from Wien’s Law. Red stars are cooler than blue stars. We can then estimate their energy output using Stefen-Boltzmann Law. In the picture to the left which star is cooler? Which star puts out more energy per unit area? Which star puts out more total energy?

Doppler Effect

The Doppler Effect If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer

The Doppler Effect Depends only on the relative motion of source and observer:

The Doppler Effect Relationship between frequency and speed:

Summary Wave: period, wavelength, amplitude Electromagnetic waves created by accelerating charges Visible spectrum is different wavelengths of light Entire electromagnetic spectrum: radio waves, infrared, visible light, ultraviolet, X rays, gamma rays

Summary, cont. Can tell the temperature of an object by measuring its blackbody radiation Doppler effect can change perceived frequency of radiation Doppler effect depends on relative speed of source and observer

Resources Chaisson & McMillan, (2002, 2004). Astronomy Today (4th & 5th Ed.) Bennett et al. (2004) The Cosmic Perspective (3rd Ed.) Shipman, Wilson, and Todd, (2003). An Introduction to Physical Science (10th Edition).