Lecture 6: The Physics of Light, Part 1. Astronomy 111

Transcription

1 Lecture 6: The Physics of Light, Part 1 Astronomy 111

2 The nature of light Look, but don t touch. - Astronomers Motto

3 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.

4 Visible light is just one form of electromagnetic radiation - but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.

5 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 (186,000 miles/sec). c = 300,000 km/s

6 Speed of light Ole Romer (Danish, ) 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!

7 Light year A light-year is the distance light travels in one year 1 light-year = 9.5 x 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

8 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.

9 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.

10 Wave Characteristics

11 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)!

12 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 ν) h = Planck s constant (A Small Number)

13 Photons Don t forget units! Wavelength -> length! Frequency -> 1/time (per second)! Energy -> joules!

14 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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16 The complete spectrum of light Gamma rays (λ < 0.01 nanometers) X-rays ( nm) Ultraviolet ( nm) Visible ( nm) Infrared (700 nm 1 mm) Microwaves (1 100 mm) Radio (> 100 mm) Energy

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

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20 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.

21 NASA's SOFIA Observatory flies a 2.7 m telescope to altitudes as high as 45,000 feet.

22 Sky: Optical

23 Sky: Infrared

24 Sky: Microwaves

25 Sky: Radio

26 Sky: X-ray

27 How light and matter interact

28 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)

29 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 of the volume is occupied.

30 Simple Atoms proton electron neutron 1 H 4 He

31 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

32 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

33 Hydrogen 1 proton Helium 2 protons 1 H 3 He 2 H 3 H 4 He Lithium 3 protons 6 Li 7 Li Proton: Neutron:

34 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 + ν e 14 C (6p+8n) 14 N (7p+7n) + e + ν e (basis of radioactive carbon dating) Free neutrons are unstable: n p + e + ν e

35 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.

36 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).

37 Emission & Absorption

38 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.

39 Absorb a Photon photon Collide with an electron

40 Absorption

41 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).

42 Emit a Photon photon Collide with an electron

43 Emission

44 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.

45 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.

46 Emission & Absorption Lines

47 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.

48 Absorb a Photon ion photon ion Collide with an electron

49 Fundamental Forces of Nature All interactions in nature are governed by 4 fundamental forces: Gravitational Force Electromagnetic Force Strong Nuclear Force Weak Nuclear Force

50 Gravitational Force Gravitation binds masses over long distances Long-range attractive force Weakest force of nature Obeys the Inverse Square Law of distance:

51 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: times stronger than Gravity.

52 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.

53 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.

54 Few closing questions: 1) Why are our eyes sensitive to visible light? 2) Could we have radio eyes? 3) Why is a leaf green?