Lecture 6: The Physics of Light, Part 1
Astronomy 111
The nature of light
“Look, but don’t touch.” - Astronomers’ Motto
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Visible light is just one form of electromagnetic radiation
The universe contains electrically charged particles: electrons (-) and protons (+). Charged particles are surrounded by electric fields and magnetic fields. Fluctuations in these fields produce electromagnetic radiation. ASTR111 Lecture 6
Visible light is just one form of electromagnetic radiation
- but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.
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Speed of light
Speed of wave, c, equals wavelength times frequency (units = meter/sec): c = λ x ν The speed of light in a vacuum is always c = 300,000 km/s (186,000 miles/sec).
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Speed of light Ole Romer (Danish, 1644-1710) was the first person to measure the speed of light
Measured timing of eclipses of Jupiter’s moon Io at different times of the year—observed that light took longer when Earth was near Jupiter’s orbit! ASTR111 Lecture 6
Light year • A light-year is the distance light travels in one year • 1 light-year = 9.5 x 1012 km • A unit of distance—not a unit of time! • For reference, – The Moon is 1.25 light-seconds from Earth – Earth is 8.3 light-minutes from the Sun – The Sun is 4.3 light-years from the nearest star
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Light can be thought of as a wave Wave = a periodic fluctuation travelling through a medium. Ocean wave = fluctuation in the height of water. Sound wave = fluctuation in air pressure. Electromagnetic wave = fluctuation in electric and magnetic fields.
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Wave Characteristics (1) Wavelength, λ (lambda): distance between wave crests (units = meter). (2) Frequency, ν (nu): number of crests passing per second (units = 1/sec = Hertz). (3) Amplitude, a: height of wave crests.
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Wave Characteristics
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Particle nature of light • Particles of light are called “photons”! • Each photon has a wavelength and a frequency! • A photon’s energy depends on its frequency (wavelength)!
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Photons
The energy of a photon is related to the frequency of a wave:
E = hf E = energy of photon f = frequency of light (also called ν) h = Planck’s constant (A Small Number) ASTR111 Lecture 6
Photons
Don’t forget units!! Wavelength -> length! Frequency -> 1/time (per second)! Energy -> joules! ASTR111 Lecture 6
Light forms a spectrum from short to long wavelength
Visible light has wavelengths from 400 to 700 nanometers. [1 nanometer (nm) = 10-9 meter]
Color is determined by wavelength: Blue: 480 nm Green: 530 nm Red: 660 nm ASTR111 Lecture 6
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The complete spectrum of light
Gamma rays (λ < 0.01 nanometers) X-rays (0.01 – 10 nm) Ultraviolet (10 – 400 nm) Visible (400 – 700 nm) Infrared (700 nm – 1 mm) Microwaves (1 – 100 mm) Radio (> 100 mm)
Energy
Visible light occupies only a tiny sliver of the full spectrum. ASTR111 Lecture 6
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Earth’s atmosphere is transparent to visible light and some microwaves and radio waves. To observe efficiently at other wavelengths, we must go above atmosphere.
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NASA's SOFIA Observatory flies a 2.7 m telescope to altitudes as high as 45,000 feet.
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Sky: Optical
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Sky: Infrared
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Sky: Microwaves
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Sky: Radio
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Sky: X-ray
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How light and matter interact
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Atoms
Ordinary matter is found primarily in the form of atoms. Range of ordinary matter: – free subatomic particles (protons & electrons) – single atoms (hydrogen, helium, gold, etc.) – simple molecules (O2, H2O) – macromolecules (DNA, complex polymers) ASTR111 Lecture 6
Atomic Structure
Nucleus of heavy subatomic particles: – proton: positively charged – neutron: uncharged (neutral)
Cloud of Electrons orbiting the Nucleus: – electron: negatively charged. – mass 1/1860th of proton
Mostly empty space 1 part in 1015 of the volume is occupied. ASTR111 Lecture 6
Simple Atoms
1H
4He
proton electron neutron ASTR111 Lecture 6
Chemical Elements
Distinguish atoms into Elements by the total number of protons in the nucleus. 1 proton = Hydrogen 2 protons = Helium 3 protons = Lithium ... and so on
Number of electrons = Number of protons (at least in conditions here on earth) Elements are Chemically Distinct ASTR111 Lecture 6
Isotopes
Distinguish elements into Isotopes by the number of neutrons in the nucleus. Example: 12C
has 6 protons and 6 neutrons 13C has 6 protons and 7 neutrons 14C has 6 protons and 8 neutrons
same # of protons & electrons, but different # of neutrons ASTR111 Lecture 6
Hydrogen 1 proton
1H
2H
3He
4He
Helium 2 protons
Lithium 3 protons
6Li Proton:
7Li Neutron:
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3H
Radioactivity
If too many or too few neutrons in a nucleus, it is unstable against radioactive decay. Examples: 3H
(1p+2n) → 3He (2p+1n) + e- + νe 14C (6p+8n) → 14N (7p+7n) + e- + ν e (basis of radioactive carbon dating)
Free neutrons are unstable: n → p + e- + νe ASTR111 Lecture 6
Energy stored in atoms and molecules emit or absorb light Consider a single, isolated atom: A nucleus, containing protons and neutrons, is surrounded by a cloud of orbiting electrons. Electrons can emit or absorb photons. ASTR111 Lecture 6
Consider hydrogen (the simplest atom): one proton, one electron
Behaviour on subatomic scales is governed by quantum mechanics. One rule of quantum mechanics: electrons can only exist in orbits of particular energy (energy is quantized). ASTR111 Lecture 6
Emission & Absorption
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Excitation
Start out in the Ground State: All electrons are in their lowest energy orbits.
To excite an electron into a higher energy orbit, you need to absorb exactly the energy difference between orbits: – absorb a photon of exactly that energy – collide with an atom or electron and get the energy from the motion of the collider. ASTR111 Lecture 6
Absorb a Photon
photon
Collide with an electron ASTR111 Lecture 6
Absorption
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De-Excitation
Excited states are unstable, and electrons will decay back into their ground states. To de-excite, an electron must rid itself of exactly the amount of excess energy: – emit a photon of the exact energy. – give up the energy to a colliding atom or electron (no photons are emitted).
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Emit a Photon
photon
Collide with an electron ASTR111 Lecture 6
Emission
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Line Spectra
• Electrons can only orbit in discrete Energy Levels. • Atoms & molecules can only emit or absorb photons at particular wavelengths. – a unique “line spectrum” for each type of atom or molecule. – what lines you see depends on the state of excitation and ionization of the system. ASTR111 Lecture 6
Emission & Absorption Lines
• Emission Lines Photons emitted at particular wavelengths when an electron jumps from a higher to a lower energy orbit.
• Absorption Lines Photons absorbed at particular wavelengths if their energy is exactly enough to make an electron jump up to a higher energy orbit. ASTR111 Lecture 6
Emission & Absorption Lines
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Ionization
If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected. Ion: positively charged atom or molecule. – Changes the spectral line signature – Changes the chemical properties
Distinguish ions by the number of electrons removed. ASTR111 Lecture 6
Absorb a Photon
ion
ion
photon
Collide with an electron ASTR111 Lecture 6
Fundamental Forces of Nature
All interactions in nature are governed by 4 “fundamental” forces: • Gravitational Force • Electromagnetic Force • Strong Nuclear Force • Weak Nuclear Force
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Gravitational Force
Gravitation binds masses over long distances • Long-range attractive force • Weakest force of nature • Obeys the Inverse Square Law of distance:
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Electromagnetic Force
Acts between charged particles: • like charges repel each other • opposite charges attract each other
Long-range, inverse-square law force. Binds: • electrons to protons in atoms • atoms to atoms in molecules
Very strong: 1040 times stronger than Gravity. ASTR111 Lecture 6
Strong & Weak Nuclear Forces
Short-range forces (