RADIATION AND SPECTRA Chapter 4

WAVESWAVES l A stone dropped into a pool of water causes an expanding disturbance called a wave.

WAVESWAVES l Light and radio are waves (called electromagnetic radiation) caused by charged particles (mostly electrons) oscillating. l A stone dropped into a pool of water causes an expanding disturbance called a wave. l Sound is a wave caused by a pressure disturbance.

PROPERTIES OF RADIATION l Speed = 3 x 10 5 km/s in vacuum. l Radiation often behaves as a wave. l Wavelengths (1nm = m) ä Radio = 1m (10 9 nm) ä Infrared = 10  m (10 4 nm) ä Visible = 0.5  m (500 nm) ä Ultraviolet = 10 nm ä X-ray = 0.1 nm    -ray = nm ä m = metre,  = 10 -6, n = 10 -9

ELECTROMAGNETIC RADIATION

ELECTROMAGNETIC WAVES

HUMAN SENSITIVITY to WAVES l Sound Waves  (wavelength) = pitch  Short = high pitch  Long = low pitch l Light Waves  (wavelength) = colour  Short = bluer  Long = redder

NANOMETERNANOMETER Usual unit of for light is nm Usual unit of for light is nm ä(nano-meter = metres) äBlue light = 400 nm äRed light = 700 nm

NANOMETERNANOMETER Usual unit of for light is nm Usual unit of for light is nm ä(nano-meter = metres) äBlue light = 400 nm äRed light = 700 nm

PROPERTIES OF RADIATION l Speed = 3 x 10 5 km/s in vacuum. l Radiation often behaves as a wave. l Wavelengths (1nm = m) ä Radio = 1m (10 9 nm) ä Infrared = 10  m (10 4 nm) ä Visible = 0.5  m (500 nm) ä Ultraviolet = 10 nm ä X-ray = 0.1 nm    -ray ä m = metre,  = 10 -6, n = l Generally the hotter the source the shorter its emitted wavelength

ELECTROMAGNETIC RADIATION Type of Radiation Wavelength Range (nm) Radiated by Objects at this Temperature Typical Sources Gamma rays Less than 0.01 More than 10 8 K Few astronomical sources this hot. Some supernovae, pulsars, black holes and gamma ray quasars. X rays KGas in clusters of galaxies; supernova remnants; solar corona Ultraviolet KSupernova remnants; very hot stars Visible KStars Infrared K Cool clouds of dust and gas; planets; satellites RadioMore than 10 6 Less than 10KNo astronomical objects this cold. Radio emission produced by electrons moving in magnetic fields (synchrotron radiation)

PROPERTIES OF RADIATION l Speed = 3 x 10 5 km/s in vacuum. l Radiation often behaves as a wave. l Wavelengths (1nm = m) ä Radio = 1m (10 9 nm) ä Infrared = 10  m (10 4 nm) ä Visible = 0.5  m (500 nm) ä Ultraviolet = 10 nm ä X-ray = 0.1 nm    -ray ä m = metre,  = 10 -6, n = l Propagation of radiation

PROPAGATION of RADIATION INVERSE SQUARE LAW

PROPERTIES OF RADIATION l Speed = 3 x 10 5 km/s in vacuum. l Radiation often behaves as a wave. l Wavelengths (1nm = m) ä Radio = 1m (10 9 nm) ä Infrared = 10  m (10 4 nm) ä Visible = 0.5  m (500 nm) ä Ultraviolet = 10 nm ä X-ray = 0.1 nm    -ray ä m = metre,  = 10 -6, n = l Propagation of radiation l Spectrum of radiation (blackbody)

WHITE LIGHT SPECTRUM

BLACKBODY RADIATION l Astronomical objects emit energy at different wavelengths

ORION CONSTELLATION Rigel Betelgeuse

BLACKBODY RADIATION l Astronomical objects emit energy at different wavelengths l Blackbody ä WHY? ä Temperature ä a source that absorbs all radiation hitting it. ä Energy is re-emitted at all wavelengths. ä Spectrum emitted depends on the temperature ä At higher temperatures, more energy is emitted.  F =  T 4 ä The higher the temperature, the shorter is the maximum wavelength.   max (nm) = 2.9 x 10 6 /T(ºK) Q ºK = ºC + 273

BLACKBODY CURVES EMITTED ENERGY WAVELENGTH (nm) | | | | | | | 3,000 K (960 nm) 4,000 K (725 nm) 5,000 K (580 nm) WIEN’S LAW T = Temp ºK max in nanometers 7,000 K (400 nm)

ORION CONSTELLATION Rigel Betelgeuse

FLASHCARDFLASHCARD WHAT IS YOUR APPROXIMATE BODY TEMPERATURE IN DEGREES K? A) 100 K B) 200 K C) 300 K D) 400 K

FLASHCARDFLASHCARD AT WHAT WAVELENGTH DO YOU PUT OUT MOST OF YOUR ENERGY? A) 100 nm (Ultra violet) B) 1000 nm (deep red) C) 10,000 nm (infrared) D) 1,000,000 nm (short radio)

DOPPLER SHIFT

Doppler Shift Formula l Change in wavelength = (original wavelength) times (v/c) l c = 300,000 km/sec l eg wavelength 400 nm from source moving ½ c away from you. l l change in wavelength = (wavelength) times (v/c) = (400) times (½) = 200 nm l l wavelength thus observed at 600 nm – colour change from blue to red

FLASHCARDFLASHCARD IMAGINE THAT YOU ARE ON A SPACESHIP, SPEEDING TOWARDS MARS (THE RED PLANET). YOU GET CONFUSED AND MISIDENTIFY IT AS EARTH (THE BLUE PLANET). HOW FAST WERE YOU GOING? (c = 3 x 10 5 km/s, blue light = 400 nm, red light = 700 nm) A) 2/7 c ( = 85,700 km/s) B) 3/7 c (= 128,570 km/s) C) 4/7 c (= 171,430 km/s) D) 5/7 c (= 214,290 km/s)