Radiation
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
Information from the Skies
Wave motion: transmits energy without the
physical transport of material
Information from the Skies
Example: water wave
Water just moves
up and down
Wave travels and
can transmit
energy
Information from the Skies
Frequency: number of wave crests that pass a
given point per second
Period: time between passage of successive
crests
Relationship:
Frequency = 1 / Period
Information from the Skies
Wavelength: distance between successive
crests
Velocity: speed at which crests move
Relationship:
Velocity = Wavelength / Period
Information from the Skies
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
1. 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).