Donna Kubik PHYS162 Fall, 2006

Because of its electric and magnetic properties, light is called electromagnetic radiation. It consists of perpendicular, oscillating electric and magnetic fields

DEMO: http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=52

Electromagnetic radiation

History of electromagnetic radiation • Theoretical prediction

– 1860’s Maxwell described electricity and magnetism with 4 equations– Described light as oscillating electric and magnetic fields– The theory placed no limit on wavelength/frequency, but light travels at about

3x108 m/s, c, called the speed of light

• Experimental observation– 1800 British astronomer William Herschel discovered infrared radiation in an

experiment with a prism. Held a thermometer beyond red light and detected a temperature, indicating it was being exposed to an invisible form of energy

– 1888 In an experiment with electric sparks, Heinrich Hertz produced EM radiation with wavelength of a few cm (radio waves)

– 1895 Wilhelm Rontgen invented a machine with wavelength shorter than 10nm, now called xrays

Light has both wave and particle properties

• Light can act as both waves and particles• Light travels as waves enclosed in discrete packets called

photons (Einstein proved by the photoelectric effect)• Photons of different wavelengths have different amounts of

energy• The shorter the wavelength, the higher a photon’s energy

At first glance, the different types of electromagnetic radiation all look alike

• They are all oscillating electric and magnetic fields

• They are all comprised of photons

• They all travel at the speed of light

But not all electromagnetic radiation is alike

Short wavelength, high energy Long wavelength, low energy

• However, each different wavelength has a different energy!

Electromagnetic radiation

We can use

wavelength or frequency or energy

to describe a specific type of electromagnetic radiation.

Wavelength

Wavelength () is the distance between two waves.

The period is the time between waves.

1 wavelength

Frequency

Frequency () is the inverse of the period.

Frequency is measured in hertz (Hz).

Speed, wavelength and frequency are related:

c =

1 wavelength

1 second

1 wavelength/sec = 1 Hz

EnergyPlanck’s Law

Planck’s Law relates frequency (or wavelength) of an electromagnetic wave to the energy of the photon

Planck’s law

E = h E=h(c/

where

E is the energy.

is the frequency. =c/=wavelength

h is Planck’s constant, h = 6.6 10-34 J s

3 equivalent ways to describe EM radiation

So there are 3 equivalent ways to describe a type of electromagnetic radiation:

wavelength

frequency

energy

3 ways to describe EM radiation

Due to “tradition”, radio astronomers tend to refer to frequency, optical astronomers tend to use wavelength, and x-ray and gamma ray astronomers prefer to use energy to describe the electromagnetic radiation they are studying

This is tabulated on the next slide:

3 ways to describe EM radiation

frequency

wavelength

energy

RadioMillimeter

Sub-millimeterInfraredOptical

UltravioletX-ray

Gamma ray

Electromagnetic radiation

• A long time ago, in a galaxy far, far away …

• An electron was moved.

• This motion caused an electromagnetic wave to be launched, which then propagated away…

• At a later time, at another locale, this wave, and many others from other electrons in the universe, arrived at a telescope and were observed by an astronomer

• The superposition of all these fields was recorded, providing information about the sources that generated the electromagnetic fields.

Electromagnetic radiation

• What can we learn about the radiating source from such observations?

Some things we can learn about the radiating source

• Blackbody radiation• The temperature of a star can be determined by the

shape of its blackbody curve.

• Spectral lines• The chemistry of an object can be determined by

identifying its spectral lines

• Doppler shift• The radial motion of a celestial object can be determined

by the Doppler shift of its spectral lines

Blackbody radiation

• A blackbody does not reflect any light; it absorbs all radiation falling onto it

• Since it reflects no electromagnetic radiation, the radiation it does emit is entirely the result of its temperature

Blackbody radiation

• An object’s temperature determines the relative number of photons that it emits at each wavelength

– As an object heats up, it gets brighter, emitting more electromagnetic radiation at all wavelengths (Stefan-Boltzman Law)

– The brightest color of the emitted radiation changes with temperature (Wein’s Law)

Blackbody radiationStefan-Boltzman Law

Kelvinin T

)m

Joules(flux energy theis

2

4

sF

TF

⋅

=σ

Blackbody radiationWein’s Law

Kelvinin T and metersin is

0029.0max

T

=

Blackbody radiationTemperature

• A star behaves almost like a perfect blackbody, so astronomers can use the Stefan-Boltzman law to relate its energy output to its surface temperature

• The temperature of a star can be determined by the shape of its blackbody curve.

DEMO: http://www-astro.phast.umass.edu/courseware/vrml/bb/bbjav.html

Blackbody radiationTemperature

• A star is considered to be an example of a "perfect radiator and perfect absorber" called a black body. 

• This is an idealized body that absorbs all electromagnetic energy incident on it. 

• Stars are good approximations to a black body, because their hot gases are very opaque, that is, the stellar material is a very good absorber of radiation.

Blackbody radiationWhat color is the Sun?

• The sun emits all colors, but it emits most strongly in the blue-green.

• Since the eye is less sensitive to blue-green than to yellow, we see the sun as yellow

Spectral lines

• The chemistry of an object can be determined by identifying its spectral lines

– Because each element produces its own unique pattern of spectral lines when an electron jumps from one energy level to another

absorption

emission

Spectral linesAbsorption and emission lines

• An absorption line is created when an electron jumps from an inner orbit to and outer orbit, extracting the required photon from an outside source of energy, such as the continuous spectrum of a hot, glowing object

• An emission line is produced when an electron transitions to a lower orbit and emits a photon

Spectral linesKirchhoff’s Laws

• Gustav Kirchhoff discovered the conditions under which continuum, emission, and absorption spectra are observed. His description is summarized as Kirchhoff’s Laws

Law 1

Law 2

Law 3

Spectral linesKirchhoff’s Laws

• Law 1 A hot object or a hot, dense gas produces a continuous spectrum (also called a continuum) - a complete rainbow without any spectral lines. This is a black body spectrum

Law 1

Law 2

Law 3

Spectral linesKirchhoff’s Laws

• Law 2 A hot-rarefied gas produces an emission line spectrum – a series of bright spectral lines against a dark background

Law 1

Law 2

Law 3

Spectral linesKirchhoff’s Laws

• Law 3 A cool gas in front of a continuous source of light produces an absorption line spectrum – a series of dark spectral lines among the colors of the rainbow

Law 1

Law 2

Law 3

Spectral linesAbsorption and emission lines

• The absorption spectrum of the Sun is an example of Kirchhoff’s Third Law.

Doppler shift

• The radial motion of a celestial object can be determined by the Doppler shift of its spectral lines

Doppler shift

• Note that only the radial velocity of a celestial object can be determined by its Doppler shift

Radial velocity

Proper motion

Doppler-shifted spectral lines

Doppler shift

• Examples of things that can be learned from measuring the Doppler shift

– Motion of hot gases on the sun

– Measurements of star motion in double star systems

– Doppler measurements of spectra of distant galaxies enable us to determine the rate at which the entire universe is expanding