RADIATION AND SPECTRARADIATION AND SPECTRARADIATION AND SPECTRARADIATION AND SPECTRA

Chapter 4Chapter 4

WAVESWAVESWAVESWAVES

A stone dropped into a pool of water causes an A stone dropped into a pool of water causes an expanding disturbance called a expanding disturbance called a wavewave..

WAVESWAVESWAVESWAVES

Light and radio are Light and radio are waveswaves (called electromagnetic (called electromagnetic radiation) caused by charged particles (mostly radiation) caused by charged particles (mostly electrons) oscillating. electrons) oscillating.

A stone dropped into a pool of water causes an A stone dropped into a pool of water causes an expanding disturbance called a expanding disturbance called a wavewave..

Sound is a Sound is a wavewave caused by a pressure disturbance. caused by a pressure disturbance.

PROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATION

Speed = 3 x 10Speed = 3 x 1055 km/s in vacuum. km/s in vacuum. Radiation often behaves as a wave. Radiation often behaves as a wave. Wavelengths (1nm = 10Wavelengths (1nm = 10-9-9 m) m)

Radio = 1m (109 nm) Infrared = 10 m (104 nm) Visible = 0.5 m (500 nm) Ultraviolet = 10 nm X-ray = 0.1 nm-ray = 10-4 nm m = metre, = 10-6, n = 10-9

ELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATION

ELECTROMAGNETIC WAVESELECTROMAGNETIC WAVESELECTROMAGNETIC WAVESELECTROMAGNETIC WAVES

HUMAN SENSITIVITY to WAVESHUMAN SENSITIVITY to WAVESHUMAN SENSITIVITY to WAVESHUMAN SENSITIVITY to WAVES

Sound WavesSound Waves (wavelength) = pitchShort = high pitchLong = low pitch

Light WavesLight Waves (wavelength) = colourShort = bluerLong = redder

NANOMETERNANOMETERNANOMETERNANOMETER

Usual unit of Usual unit of for light is nm for light is nm(nano-meter = 10-9 metres)Blue light = 400 nmRed light = 700 nm

NANOMETERNANOMETERNANOMETERNANOMETER

Usual unit of Usual unit of for light is nm for light is nm(nano-meter = 10-9 metres)Blue light = 400 nmRed light = 700 nm

PROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATION Speed = 3 x 10Speed = 3 x 1055 km/s in vacuum. km/s in vacuum. Radiation often behaves as a wave. Radiation often behaves as a wave. Wavelengths (1nm = 10Wavelengths (1nm = 10-9-9 m) m)

Radio = 1m (109 nm) Infrared = 10 m (104 nm) Visible = 0.5 m (500 nm) Ultraviolet = 10 nm X-ray = 0.1 nm-ray m = metre, = 10-6, n = 10-9

Generally the hotter the source the shorter its emitted Generally the hotter the source the shorter its emitted wavelengthwavelength

ELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONType of Type of

RadiationRadiationWavelength Wavelength Range (nm)Range (nm)

Radiated by Radiated by Objects at this Objects at this TemperatureTemperature

Typical SourcesTypical Sources

Gamma rays Less than 0.01

More than 108 K Few astronomical sources this hot. Some supernovae, pulsars, black holes and gamma ray quasars.

X rays 0.01 - 20 106 - 108 K Gas in clusters of galaxies; supernova remnants; solar corona

Ultraviolet 20-400 104 - 106 K Supernova remnants; very hot stars

Visible 400-700 103 - 104 K Stars

Infrared 103 - 106 10 - 103 K Cool clouds of dust and gas; planets; satellites

Radio More than 106 Less than 10K No astronomical objects this cold. Radio emission produced by electrons moving in magnetic fields (synchrotron radiation)

PROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATION

Speed = 3 x 10Speed = 3 x 1055 km/s in vacuum. km/s in vacuum. Radiation often behaves as a wave. Radiation often behaves as a wave. Wavelengths (1nm = 10Wavelengths (1nm = 10-9-9 m) m)

Radio = 1m (109 nm) Infrared = 10 m (104 nm) Visible = 0.5 m (500 nm) Ultraviolet = 10 nm X-ray = 0.1 nm-ray m = metre, = 10-6, n = 10-9

Propagation of radiationPropagation of radiation

PROPAGATION of RADIATIONPROPAGATION of RADIATIONPROPAGATION of RADIATIONPROPAGATION of RADIATION

INVERSE SQUARE LAW

PROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATIONPROPERTIES OF RADIATION Speed = 3 x 10Speed = 3 x 1055 km/s in vacuum. km/s in vacuum. Radiation often behaves as a wave. Radiation often behaves as a wave. Wavelengths (1nm = 10Wavelengths (1nm = 10-9-9 m) m)

Radio = 1m (109 nm) Infrared = 10 m (104 nm) Visible = 0.5 m (500 nm) Ultraviolet = 10 nm X-ray = 0.1 nm-ray m = metre, = 10-6, n = 10-9

Propagation of radiationPropagation of radiation Spectrum of radiation (blackbody)Spectrum of radiation (blackbody)

WHITE LIGHT SPECTRUMWHITE LIGHT SPECTRUMWHITE LIGHT SPECTRUMWHITE LIGHT SPECTRUM

BLACKBODY RADIATIONBLACKBODY RADIATIONBLACKBODY RADIATIONBLACKBODY RADIATION

Astronomical objects emit energy at different Astronomical objects emit energy at different wavelengthswavelengths

ORION CONSTELLATIONORION CONSTELLATIONORION CONSTELLATIONORION CONSTELLATION

Rigel

Betelgeuse

BLACKBODY RADIATIONBLACKBODY RADIATIONBLACKBODY RADIATIONBLACKBODY RADIATION

Astronomical objects emit energy at different Astronomical objects emit energy at different wavelengthswavelengths

Blackbody Blackbody

WHY?WHY? TemperatureTemperature

a source that absorbs all radiation hitting it.a source that absorbs all radiation hitting it. Energy is re-emitted at all wavelengths.Energy is re-emitted at all wavelengths. Spectrum emitted depends on the temperatureSpectrum emitted depends on the temperature At higher temperatures, more energy is emitted.At higher temperatures, more energy is emitted.

F = F = TT44

The higher the temperature, the shorter is the The higher the temperature, the shorter is the maximum wavelength.maximum wavelength.maxmax(nm) = 2.9 x 10(nm) = 2.9 x 1066 /T(ºK) /T(ºK) ºK = ºC + 273ºK = ºC + 273

BLACKBODY CURVESBLACKBODY CURVESBLACKBODY CURVESBLACKBODY CURVESE

MIT

TE

D E

NE

RG

Y

WAVELENGTH (nm)0 500 1000 1500 2000 2500 3000

| | | | | | |

3,000 K(960 nm)

4,000 K(725 nm)

5,000 K(580 nm)

WIEN’S LAW

x

max 2.9x106

T

T = Temp ºKmax in nanometers

7,000 K(400 nm)

ORION CONSTELLATIONORION CONSTELLATIONORION CONSTELLATIONORION CONSTELLATION

Rigel

Betelgeuse

FLASHCARDFLASHCARDFLASHCARDFLASHCARD

WHAT IS YOUR APPROXIMATE BODY WHAT IS YOUR APPROXIMATE BODY TEMPERATURE IN DEGREES K?TEMPERATURE IN DEGREES K?

A)A) 100 K100 K

B)B) 200 K 200 K

C)C) 300 K 300 K

D)D) 400 K 400 K

FLASHCARDFLASHCARDFLASHCARDFLASHCARD

AT WHAT WAVELENGTH DO YOU PUT OUT AT WHAT WAVELENGTH DO YOU PUT OUT MOST OF YOUR ENERGY?MOST OF YOUR ENERGY?

A)A) 100 nm (Ultra violet)100 nm (Ultra violet)

B)B) 1000 nm (deep red) 1000 nm (deep red)

C)C) 10,000 nm (infrared) 10,000 nm (infrared)

D)D) 1,000,000 nm (short radio) 1,000,000 nm (short radio)

DOPPLER SHIFTDOPPLER SHIFTDOPPLER SHIFTDOPPLER SHIFT

Doppler Shift FormulaDoppler Shift FormulaDoppler Shift FormulaDoppler Shift Formula

Change in wavelength = (original wavelength) times Change in wavelength = (original wavelength) times (v/c)(v/c) c = 300,000 km/sec c = 300,000 km/sec eg wavelength 400 nm from source moving ½ c away eg wavelength 400 nm from source moving ½ c away

from you.from you. change in wavelength = (wavelength) times (v/c) =

(400) times (½) = 200 nm wavelength thus observed at 600 nm – colour change

from blue to red

FLASHCARDFLASHCARDFLASHCARDFLASHCARD

IMAGINE THAT YOU ARE ON A SPACESHIP, IMAGINE THAT YOU ARE ON A SPACESHIP, SPEEDING TOWARDS MARS (THE RED SPEEDING TOWARDS MARS (THE RED PLANET). YOU GET CONFUSED AND PLANET). YOU GET CONFUSED AND MISIDENTIFY IT AS EARTH (THE BLUE PLANET). MISIDENTIFY IT AS EARTH (THE BLUE PLANET). HOW FAST WERE YOU GOING? HOW FAST WERE YOU GOING? (c = 3 x 10(c = 3 x 1055 km/s, blue light = 400 nm, red light = 700 nm) km/s, blue light = 400 nm, red light = 700 nm)

A)A) 2/7 c ( = 85,700 km/s)2/7 c ( = 85,700 km/s)

B)B) 3/7 c (= 128,570 km/s) 3/7 c (= 128,570 km/s)

C)C) 4/7 c (= 171,430 km/s) 4/7 c (= 171,430 km/s)

D)D) 5/7 c (= 214,290 km/s) 5/7 c (= 214,290 km/s)