quiz 1 each quiz sheet has a different 5-digit symmetric number which must be filled in (as shown on...
TRANSCRIPT
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Quiz 1• Each quiz sheet has a different 5-digit
symmetric number which must be filled in (as shown on the transparency, but NOT the same one!!!!!)
• Please hand in both the exam and the answer sheets with your name on both
• Question/answer sheets will be handed back on Wednesday after class
• Please remain seated until we begin collecting (20-25 minutes after start)
• Class after quiz
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Why is the sky blue ?
The atmosphere scatters the blue light more than red light
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Light and Matter• Light is electromagnetic energy, due to
interaction of electrical charges• Matter is made of atoms – equal number
of positive and negative particles• An atom is the smallest particle of an
element; natural element H to U• Atom Nucleus (protons + neutrons),
with ‘orbiting’ electrons• No. of protons in nucleus = Atomic
Number• Science of light Spectroscopy
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Radiation and Spectroscopy
• Light is electromagnetic energy
• Propagates as both particles and waves
• Photons – particles of light
• Wavelength = Velocity / Frequency
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Light is electromagnetic wave;Does not require a medium to propagate,
unlike water or sound
Wavelength is the distance between successive crests or troughs
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Wavelength ()
Speed (c)
Frequency (f) (# waves/second)
Speed = wavelength x frequency c = fFrequency ‘f’ is the number of waves passing a point per second
WAVES: Frequency, Wavelength, Speed
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Units of wavelength and frequency
• Frequency is the number of cycles per second• Since speed of light is constant, higher the
frequency the shorter the wavelength and vice-versa
• Wavelengths are measured in Angstroms: 1A = 1/100,000,000 cm = 1/10 nanometer (nm)
• The higher the frequency the more energetic the wave
• Wavelength (or frequency) defines radiation or color
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Spectrum
Prism
WhiteLight
Prisms disperse light into its component colors: Red-Violet
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Visible Light
• Forms a narrow band within the electromagnetic spectrum ranging from gamma rays to radio waves
• Human eye is most sensitive to which color?
• Yellow. Why?
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Light: Electromagnetic SpectrumFrom Gamma Rays to Radio Waves
Gamma rays are the most energetic (highest frequency, shortest wavelength),Radio waves are the least energetic.
Gamma
X-Ray
UV
Visible
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DecreasingWavelength ORIncreasingFrequency
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Visible light spectrum: Each color is defined by its wavelength, frequency or energy
Red - Blue 7000 - 4000 Angstroms ( 1 nm = 10 A, 1 A = 10-8 cm)Blue light is more energetic than red light
Light also behaves like ‘particles’ called photons Photon energy, frequency, wavelength: E = h f = hc/ Planck’s Law (‘h’ is a number known as Planck’s constant)
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Matter and Particles of Light: Quantum Theory
• Light (energy) and matter in motion behave both as waves and particles
• Wave-Particle Duality - Quantum Theory • Particles of light are called photons: E = hf = hc/• Photons of a specific wavelength may be absorbed
or emitted by atoms in matter• Matter is made of different natural elements: lightest
Hydrogen (1 proton), heaviest Uranium (92 protons)• Smallest particle of an element is atom, made up of a
nucleus (protons and neutrons), and orbiting electrons• Electrons and protons attract as opposite electrical
charges, NOT gravitationally like planets and Sun
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The Hydrogen Atom
Electron orbitsDiscrete energies
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Absorption of light (energy) photon by H-atom
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Emission of light photon by H-atomphoton energy color
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Series of spectral lines of Hydrogen
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Wavelengths of series of lines from Hydrogen
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SPECTRAL SIGNATURE OF ELEMENTS
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Continuous, Absorption, and Emission Spectra
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Brightness and Temperature
• Brightness is related to the total energy emitted, or the luminosity of an object
• The energy emitted is related to the temperature of the object
• B = T4 is a constant)
Stefan-Boltzmann Law
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Color Indicates Temperature and Energy of the Source
Objects generally emit radiation at all wavelengths, but mostly at one peak Wavelength depending on their temperature (e.g. blue – hot, red – cool)
Surface T (Sun) = 5600 K “ (Mercury) = 800 K
Blackbody: Perfectabsorber and emitterOf radiation at a givenTemperature T
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TEMPERATURE SCALES
Room Temp = 300 K = 27 C = 81 F
Astronomers usually use the Kelvin Scale
K = C + 273 C = (F - 32) x 5/9 ~ (F - 30) / 2 F = (C x 9/5) + 32 ~ C x 2 + 30
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The Doppler Effect
• Why does the “pitch” of a police siren differ when, say, a police car is approaching you, or when you are running away from the police (not recommended) ?
• The frequency (the number of sound waves per second) is higher when approaching, and smaller when receding from the source
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Doppler Effect in Sound
High Pitch(short waves)
Low Pitch(long waves)
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d=1
d=2
d=3
B=1
B=1/9
B=1/4
Brightness decreases inversely as the square of the distance
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The Doppler Effect
Velocity c = frequency (f) x wavelength (
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Doppler Shift of Wavelengths
• What about the wavelength?• What about light?• Shorter wavelength Blue-shift, • Longer wavelength Red-shift• We can determine the velocity of
astronomical objects, moving away or towards the Earth, by measuring the wavelength of light from the object
• Observed red-shift of galaxies all over the sky shows that galaxies are moving away from one another the Universe is expanding (Hubble’s Law)
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Hubble Diagram: Distribution of Galaxies
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Hubble’s Law: v = HodVelocity increases with distance
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Expanding Universe
• Hubble’s law Universe is expanding
• Universe had a beginning !
• How long ago?
• Age of the universe: 1/Ho (units of time)
• Big Bang !!
• How does one determine distances?
Redshift
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Cosmic Microwave Background (CMB): Universal and Uniform Radiation
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CMB Properties• The entire universe is filled with extremely
uniform radiation
• CMB radiation corresponds to a fixed temperature of 2.73 K (-270.3 oC or -428.9 oF)
• Blackbody: uniform temperature oven
• CMB radiation is also isotropic same in all directions
• But with extremely slight variations immediately following the Big Bang due to matter
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Distribution of Matter in Galaxy
• Stars rotate about the center of galaxy
• Velocity determined by gravity: mass Mc and distance Rc from the center
• KE = PE
• ½ mstar v2 = G Mc mstar / Rc
• Velocity v should decrease with radius Rc
• Surprise !
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Evidence of Dark Matter: Rotation Curves of Galaxies
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Dark Matter Halo
• Rotation curves are flat out to distances beyond observable galaxies
• Ergo: Galaxies have “dark matter” haloes
• What is dark matter?