RADIATION AND SPECTRA

RADIATION AND SPECTRA Chapter 4

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

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

PROPERTIES OF RADIATION • Speed = 3 x 105 km/s in vacuum. • Radiation often behaves as a wave. • Wavelengths (1nm = 10-9 m) • Radio = 1m (109 nm) • Infrared = 10 m (104 nm) • Visible = 0.5  m (500 nm) • Ultraviolet = 10 nm • X-ray = 0.1 nm • g-ray = 10-4 nm • m = metre,  = 10-6, n = 10-9

ELECTROMAGNETIC RADIATION

ELECTROMAGNETIC WAVES

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

NANOMETER • Usual unit of l for light is nm • (nano-meter = 10-9 metres) • Blue light = 400 nm • Red light = 700 nm

PROPERTIES OF RADIATION • Speed = 3 x 105 km/s in vacuum. • Radiation often behaves as a wave. • Wavelengths (1nm = 10-9 m) • Radio = 1m (109 nm) • Infrared = 10 m (104 nm) • Visible = 0.5  m (500 nm) • Ultraviolet = 10 nm • X-ray = 0.1 nm • g-ray • m = metre,  = 10-6, n = 10-9 • Generally the hotter the source the shorter its emitted wavelength

ELECTROMAGNETIC RADIATION

PROPERTIES OF RADIATION • Speed = 3 x 105 km/s in vacuum. • Radiation often behaves as a wave. • Wavelengths (1nm = 10-9 m) • Radio = 1m (109 nm) • Infrared = 10 m (104 nm) • Visible = 0.5  m (500 nm) • Ultraviolet = 10 nm • X-ray = 0.1 nm • g-ray • m = metre,  = 10-6, n = 10-9 • Propagation of radiation

PROPAGATION of RADIATION INVERSE SQUARE LAW

PROPERTIES OF RADIATION • Speed = 3 x 105 km/s in vacuum. • Radiation often behaves as a wave. • Wavelengths (1nm = 10-9 m) • Radio = 1m (109 nm) • Infrared = 10 m (104 nm) • Visible = 0.5  m (500 nm) • Ultraviolet = 10 nm • X-ray = 0.1 nm • g-ray • m = metre,  = 10-6, n = 10-9 • Propagation of radiation • Spectrum of radiation (blackbody)

WHITE LIGHT SPECTRUM

BLACKBODY RADIATION • Astronomical objects emit energy at different wavelengths

Betelgeuse Rigel ORION CONSTELLATION

BLACKBODY RADIATION • Astronomical objects emit energy at different wavelengths • WHY? • Temperature • Blackbody • a source that absorbs all radiation hitting it. • Energy is re-emitted at all wavelengths. • Spectrum emitted depends on the temperature • At higher temperatures, more energy is emitted. • F = sT4 • The higher the temperature, the shorter is the maximum wavelength. • lmax(nm) = 2.9 x 106 /T(ºK) • ºK = ºC + 273

7,000 K (400 nm) 5,000 K (580 nm) 4,000 K (725 nm) 3,000 K (960 nm) BLACKBODY CURVES WIEN’S LAW T = Temp ºK max in nanometers EMITTED ENERGY | | | | | | | 0 500 1000 1500 2000 2500 3000 WAVELENGTH (nm)

Betelgeuse Rigel ORION CONSTELLATION

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

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) FLASHCARD

DOPPLER SHIFT

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

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 105 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) FLASHCARD

RADIATION AND SPECTRA