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 (