Lecture 6: The Physics of Light, Part 1 Astronomy 111 Wednesday September 13, 2017
Reminders Star party tonight! Homework #3 due Monday Exam #1 Monday, September 25
The nature of light Look, but don t touch. - Astronomers Motto
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.
Visible light is just one form of electromagnetic radiation - but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.
Speed of light Speed of wave, c, equals wavelength times frequency (units = meter/sec): c = l x n The speed of light in a vacuum is always (186,000 miles/sec). c = 300,000 km/s
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 10 12 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
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.
Wave characteristics (1) Wavelength, l (lambda): distance between wave crests (units = meter). (2) Frequency, f or n (nu): number of crests passing per second (units = 1/sec = Hertz). (3) Amplitude, a: height of wave crests.
Wave characteristics ASTR111 Lecture 6
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)
Photons The energy of a photon is related to the frequency of a wave: E = energy of photon E = hf f = frequency of light (also called n) h = Planck s constant (A Small Number)
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 wavelengths 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
The complete spectrum of light Gamma rays (l < 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.
Earth s atmosphere is transparent to visible light and some microwaves and radio waves. ASTR111 Lecture 6
To observe efficiently at other wavelengths, we must go above atmosphere. ASTR111 Lecture 6
NASA's SOFIA Observatory flies a 2.7 m telescope to altitudes as high as 45,000 feet.
Sky: Optical ASTR111 Lecture 6
Sky: Infrared ASTR111 Lecture 6
Sky: Microwaves ASTR111 Lecture 6
Sky: Radio ASTR111 Lecture 6
Sky: X-ray ASTR111 Lecture 6
How light and matter interact ASTR111 Lecture 6
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 (O 2, H 2 O) macromolecules (DNA, complex polymers)
Atomic structure Nucleus of heavy subatomic particles: proton: positively charged neutron: uncharged (neutral) Cloud of electrons orbiting the nucleus: electron: negatively charged mass 1/1860 th of proton Mostly empty space 1 part in 10 15 of the volume is occupied
Simple atoms proton electron neutron 1 H 4 He
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
Isotopes Distinguish elements into isotopes by the number of neutrons in the nucleus. Example: 12 C has 6 protons and 6 neutrons 13 C has 6 protons and 7 neutrons 14 C has 6 protons and 8 neutrons Same # of protons & electrons, but different # of neutrons
Hydrogen 1 proton 1 H 2 H 3 H Helium 2 protons 3 He 4 He Lithium 3 protons 6 Li 7 Li Proton: Neutron:
Radioactivity If too many or too few neutrons in a nucleus, it is unstable against radioactive decay. Examples: 3 H (1p+2n) 3 He (2p+1n) + e - + n e 14 C (6p+8n) 14 N (7p+7n) + e - + n e (basis of radioactive carbon dating) Free neutrons are unstable: n p + e - + n e
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.
Consider hydrogen (the simplest atom): one proton, one electron Behavior 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).
Emission & absorption ASTR111 Lecture 6
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.
Absorb a photon photon Collide with an electron
Absorption ASTR111 Lecture 6
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).
Emit a photon photon Collide with an electron
Emission ASTR111 Lecture 6
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.
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.
Emission & absorption lines ASTR111 Lecture 6
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.
Absorb a photon ion photon ion Collide with an electron
Fundamental forces of nature All interactions in nature are governed by 4 fundamental forces: Gravitational Force Electromagnetic Force Strong Nuclear Force Weak Nuclear Force
Gravitational force Gravitation binds masses over long distances Long-range attractive force Weakest force of nature Obeys the Inverse-Square Law of distance: F = 1 2 2 G m m d
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: 10 40 times stronger than Gravity.
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.
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.
Questions: 1) Why are our eyes sensitive to visible light? 2) Could we have radio eyes? 3) Why is a leaf green?