Feb 15 A li*le review, Doppler shi4, and Solar System
Ocean wave = fluctuaaon in height of water. Sound wave = fluctuaaon in air pressure. ElectromagneAc wave = fluctuaaon in electric and magneac fields.
Wavelength, Frequency, c
The speed of a wave equals wavelength Ames frequency. c for celeritas, the LaAn word for speed The speed of light in a vacuum is ALWAYS c = 300,000 km/sec (186,000 miles/sec).
Figure 4.6: Wavelength, Speed, Frequency, and Amplitude
Light is made of PARTICLES. Light shows some properaes of paracles, such as the photoelectric effect. ParAcles of light, called PHOTONS, kick electrons out of atoms.
The ENERGY of a photon is related to the FREQUENCY of the wave. E = energy of photon f = frequency of light wave h = Planck s constant (a number)
Bright at all wavelengths Bright at specific wavelengths Bright at all except some specific wavelengths
An example of a ConAnuum Spectrum: Blackbody A blackbody is an object that absorbs all light that falls on it It radiates light which has a disancave spectrum, specified by its temperature The peak of the spectrum for blackbodies with T=98.6 F is in the infrared part of the spectrum T=6000 degree black bodies have peaks in the visible part of the spectrum (e.g. the SUN) Incandescent light bulb filament: 2000-3000 deg.
Stars have spectra that are more or less like those of blackbodies
Absorp'on and Emission Lines in Spectra: Produced when light interacts with atoms A nucleus, consisang of protons and (usually) neutrons, is surrounded by a cloud of electrons.
HYDROGEN: One Proton, one electron. proton electron Behavior on subatomic scales is governed by QUANTUM MECHANICS. Rule: electrons can only exist in orbits of paracular energy. (Small orbit = low energy, big orbit = high energy).
Electron falls from high energy to low energy orbit: energy is carried away by a photon. λ=656.3 nm 121.6 nm 102.6 nm Photon has a fixed ENERGY, corresponding to a specific WAVELENGTH.
Consider a hot, low density glob of hydrogen gas. λ = 656.3 nm (3 2) λ = 486.1 nm (4 2) λ = 434.0 nm (5 2) Light is emi*ed with wavelengths or frequencies corresponding to energy jumps between electron orbits.
1) Hot, low density gas produces an emission line spectrum. 5 2 4 2 3 2 Spectrum of hydrogen at visible wavelengths.
Carina Nebula: a cloud of hot, low density gas about 7000 light- years away. Its reddish color comes from the 656.3 nm emission line of hydrogen.
A cool, low density glob of hydrogen gas in front of a light source. Light is absorbed at specific wavelengths corresponding to energy jumps between electron orbits.
2) Cool, low density gas produces an absorp3on line spectrum. 5 2 4 2 3 2 Spectrum of hydrogen at visible wavelengths.
Every type of atom has a unique spectrum.
The spectrum of the Carina Nebula: Hydrogen line at 656.3 nm
The RADIAL VELOCITY of an object is found from the DOPPLER SHIFT of its spectrum. Radial velocity = how fast an object is moving toward you or away from you.
ChrisAan Doppler (1803-1853) DOPPLER SHIFT: If the wave source moves toward you or away from you, the wavelength changes.
Figure 4.16: The Doppler shi4 of light
If a light source is moving TOWARDS you, the wavelength appears shorter (called blueshi4 ). If the light source is moving AWAY from you, wavelength is longer (called redshi4 ).
Doppler shi4s are easily detected in emission or absorpaon line spectra.
Size of Doppler shi4 is proporaonal to radial velocity: Δλ = observed wavelength shi4 = λ- λ 0 λ 0 = wavelength if source isn t moving V = radial velocity of moving source c = speed of light = 300,000 km/sec
One of the ni4y applicaaons of the Doppler effect has been the detecaon of Planets orbiang other stars: Extrasolar Planets The star and planet orbit around their center of mass periodic red and blue shi4s of the lines in the star s spectrum To date: about 358 planets have been discovered, mostly larger than Jupiter
Another ni4y applicaaon of the Doppler effect : Doppler radar to predict weather Not so ni4y: Police radar guns to catch you speeding
Spectrum of the Sun
X- ray spectrum
Summary Astronomers take images of objects Astronomers also take spectra of objects Temperature Type of atoms (hydrogen, helium, iron, etc) how fast the object is moving, at least radially SomeAmes astronomers take images though filters which isolate specific wavelengths (rough) spectral pictures
IR Light The 10 micron camera contains a detector which is sensiave to infrared light. All objects radiate "black body" radiaaon, or "Planck radia'on", by virtue of their temperature.
For objects near room temperature the radiaaon peaks in the infrared. Your eyes, which are sensiave to opacal light, cannot see this radiaaon unless the object is VERY hot. The visible or opacal light you see is reflected opacal light from the sun or lamps. Ho*er things are brighter in the IR camera than cooler things. Some materials are opaque to IR light, but transparent to visible light. Some materials are transparent to IR light, but opaque to visible light. IR light can be reflected by a mirror, just like opacal light.
Everyday Uses of IR light One everyday use of IR light is in remote control devices IR cameras are used on ships and in buildings to look for hot spots in electrical wiring night- Ame sposng of people (who are warmer than their surroundings) for seeing "through" smoke in a fire
Astronomers use IR light To measure temperatures; also to look "through" dust OpAcal or Visible Wavelengths IR wavelengths The Trapezium in Orion: Stars are forming out of gas and dust
Where to put a Telescope? Far away from civilization to avoid light pollution