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 PropertiesExample: water wave

Water just moves up and down

Wave travels and can transmit energy

Wave PropertiesFrequency: number of wave crests that pass a given point per second

Period: time between passage of successive crests

Relationship:

Frequency = 1 / Period

Wave PropertiesWavelength: 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 RadiationTemperature 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 RadiationRadiation Laws

Wien’s Law

1.Peak wavelength is inversely proportional to temperature,

p 1/T.

or

Peak frequency is proportional to the temperature,

fp T.

Thermal Radiation

Radiation Laws

Stefan-Boltzmann Law

2. Total energy emitted is proportional to fourth power of temperature (note intensity of curves)

E 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 T4.

• Theories are models that explain why phenomena occur, or why laws are obeyed.

• So E 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

• 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

Summary, cont.

Resources

Chaisson & McMillan, (2002, 2004). Astronomy Today (4th & 5th Ed.)

Bennett et al. (2004) The Cosmic Perspective (3rd Ed.)

Shipman, Wilson, and Todd, (2003). AnIntroduction to Physical Science (10th

Edition).