Lecture 6: The Physics of Light, Part 1

Astronomy 111

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“Look, but don’t touch.” - Astronomers’ Motto

The nature of light

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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.

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- but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.

Visible light is just one form of electromagnetic radiation

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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!

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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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•  Particles of light are called “photons”!

•  Each photon has a wavelength and a frequency!

•  A photon’s energy depends on its frequency (wavelength)!

Particle nature of light

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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)

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Don’t forget units!!

Wavelength -> length!

Frequency -> 1/time (per second)!

Energy -> joules!

Photons

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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

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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

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Visible light occupies only a tiny sliver of the full spectrum.

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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)

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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.

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Simple Atoms

proton electron neutron

1H 4He

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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

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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

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Hydrogen 1 proton

Helium 2 protons

Lithium 3 protons

Proton: Neutron:

1H

3He

2H 3H

4He

6Li 7Li

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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- + νe

(basis of radioactive carbon dating)

Free neutrons are unstable: n → p + e- + νe

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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.

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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).

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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.

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Absorb a Photon

Collide with an electron

photon

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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

Collide with an electron

photon

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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.

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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.

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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.

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Absorb a Photon

Collide with an electron

ion photon

ion

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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.

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Strong & Weak Nuclear Forces

Short-range forces (<10-15 m) in atomic nuclei

Strong Force: – binds protons & neutrons into nuclei. – strongest force of nature.

Weak Force: –  responsible for radioactivity (turns neutron

into a proton) – second weakest force.

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Interplay of Forces

Gravity rules on the largest scales. Electromagnetism rules on intermediate

scales (atomic scales up to people) Strong & Weak Forces rule on nuclear

scales.

We will explore the different roles of each in our study of stars, galaxies & the Universe.

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Few closing questions:

1) Why are our eyes sensitive to “visible” light?

2) Could we have radio eyes? 3) Why is a leaf green?