Light! Lecture 3, Oct. 8! Astronomy 102, Autumn 2009! Oct. 8, 2009 #1. Astronomy 102, Autumn 2009, E. Agol & J. Dalcanton U.W.
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1 Light! Lecture 3, Oct. 8! Astronomy 102, Autumn 2009! Oct. 8, 2009 #1
2 Questions of the Day! I. What is light?! II. What are the wave/particle properties of light?! III. How do energy and wavelength vary along the electromagnetic spectrum?! IV. What makes light spectra interesting?! V. What are the properties of a thermal (blackbody) spectrum?! Oct. 8, 2009 #2
3 I. What is light?! Light is a form of energy! (Light carries energy from the fire to your skin which is then transformed into heat)! Light is comprised of electric & magnetic fields which carry the energy! Oct. 8, 2009 #3
4 II. The Facts of Light:! A) Light acts like a massless particle: photon! Photons:! 1. go in a straight line unless they are redirected or absorbed.! 2. are discrete objects.! 3. apply a force when they hit something.! 4. can travel through empty space! Oct. 8, 2009 #4
5 Proof that photons exist = X-ray images. This X-ray image of the moon shows individual photons detected:! Oct. 8, 2009 #5
6 B) Light also acts like a wave.! As light travels, there are high points and low points in the electric field and magnetic field which makes it behave like a wave.! Oct. 8, 2009 #6
7 So, light is a particle AND a wave!! Sometimes scientists talk about light as if it is a particle:! Speed! Energy! But other times they talk about light as a wave.! Wavelength! Frequency! Oct. 8, 2009 #7
8 III. The Characteristics of light! A. Wavelength: The distance over which the wave pattern repeats itself.! Amplitude! λ = lambda = the universal symbol for wavelength! Are we on the same λ?! Oct. 8, 2009 #8
9 Wavelength:! The color of optical light depends upon its wavelength.! Short wavelengths are! Purple or Blue! Long wavelengths are! Red! Wavelength is measured in units of nm or nanometers (10-9 m)! Oct. 8, 2009 #9
10 Wavelength:! We see with our eyes only a tiny fraction of the range of possible wavelengths! Size of atomic nucleus! Size of Mt. Everest! Note: each mark represents a factor of ten increase.! Oct. 8, 2009 #10
11 Wavelength:! Oct. 8, 2009 #11
12 Wavelength:! Natural light is comprised of many wavelengths of light! We can distinguish the different wavelengths using a prism which creates a spectrum.! Oct. 8, 2009 #12
13 B. Frequency =! the number of peaks (or troughs) that go by each second! Frequency is measured in Hertz, which has units of 1/second.! One peak in one second is 1 Hertz, Two peaks in one second is 2 Hertz, and so on...! f is the symbol for frequency! Oct. 8, 2009 #13
14 C. Speed! The speed of light is constant =! 3x10 5 km/s! The speed of light is related to its wavelength and frequency:! speed = c = distance between peaks time between peaks OR! c = λ (1/f ) = λf If you know the frequency, you can calculate the wavelength, or vice versa! Oct. 8, 2009 #14
15 Match the items from each column:! X-ray! Optical! Radio! 500 nm! 1 m! 10 nm! Hz! Hz! Hz! Oct. 8, 2009 #15
16 D. Energy! The energy of a photon is determined by this formula:! E photon = h f = hc λ h = x joule x s = Planck s constant! Energy is! greater when..! FREQUENCY! WAVELENGTH! Oct. 8, 2009 #16
17 BLUE! Short wavelength! High frequency! HIGH ENERGY! E photon = h f = hc λ RED! Long wavelength! Low frequency! LOW ENERGY! Dangerous - burns!! Oct. 8, 2009 #17 Mostly Harmless.!
18 IV. Why are spectra interesting?! Spectra give us the information we need to learn about our Universe s size, history, composition, movement, etc...! Oct. 8, 2009 #18
19 Without spectra we wouldn t know:! 1) that the Universe is expanding from the Big Bang! 2) about the existence of planets around other stars! 3) the chemical composition of any star or galaxy! 4) how much stars weigh! Oct. 8, 2009 #19
20 V. Thermal radiation spectrum:! All matter in the universe emits light due to the motion of atoms on the microscopic scale! Hot, so it emits light!
21 Rules of thermal radiation:! λ max = m T (Kelvin) Oct. 8, 2009 #21
22 Thermal radiation distribution:! If its hotter, its BLUER.! If its hotter, there s MORE of it BRIGHTER! HIGH ENERGY! (Log-log plot)! LOW ENERGY!
23 Thermal radiation distribution:! This spectral shape is closely related to the distribution of particle velocities.! Linear version of the logarithmic plot below.! P.S. Note that the plots on the right have energy decreasing to the right, whereas the left hand plot is opposite!
24 If humans have thermal radiation why don t we glow in the dark?! a) People do not emit any kind of light.! b) People only emit light that is invisible to our eyes.! c) People are too small to emit enough light for us to see.! d) People do not contain enough radioactive material.! e) People are not opaque enough to emit light.! Oct. 8, 2009 #24
25 Thermal radiation from people!! Humans emit light in the infrared.! PS. You can only see other people when there is light to reflect off of them. We glow only in the infrared!! Oct. 8, 2009 #25
26 More rules of thermal radiation:! c) Cooler objects can sometimes emit more light overall (i.e. larger power released).! Decreasing temperature means less light emitted per unit area. An object can compensate by being BIGGER.! Hot.! Cool.! Same total luminosity! Cool, but big.! Oct. 8, 2009 #26
27 d) A thermal spectrum is emitted only if the object is OPAQUE.! Thermal radiation is created by multiple interactions between photons and matter.! If photons go sailing through the matter (i.e. transparent), no thermal emission.! Wheeee! That was easy! Finally! OPAQUE! Oct. 8, 2009 #27
28 Why thermal radiation matters for astronomy! You can tell a galaxy s or star s or temperature just by measuring its color!!!!! Hot!! Cool!! Oct. 8, 2009 #28
29 A.! Which star will look bluest & peak at the shortest wavelength?! T = 5,000 K R = 1 R! B.! T = 20,000 K R = 1 R! C.! T = 5,000 K R = 10 R! D.! T = 10,000 K R = 10 R! Oct. 8, 2009 #29
30 Recap:! Light behaves like a particle & a wave! The speed of light is constant! The wavelength times frequency equals the speed: c = λf! λ = wavelength, f = frequency! c = 3.00 x 10 8 m/s = speed of light! The energy of light increases with frequency:! E = h x f = photon energy! Thermal (blackbody) radiation becomes bluer & brighter (per unit area) as the temperature increases! Oct. 8, 2009 #30
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What can we learn from light? Temperature Energy Chemical Composition Speed towards or away from us All from the spectrum! Temperature and Radiation Why do different objects give off different forms of
Energy and the Quantum Theory
Energy and the Quantum Theory Light electrons are understood by comparing them to light 1. radiant energy 2. travels through space 3. makes you feel warm Light has properties of waves and particles Amplitude:
Nature of Light. Objectives. What is light What are the different forms
Nature of Light Objectives What is light What are the different forms Light s Importance Light contributes 99% of all observations in Astronomy Light is a form of energy Light is electromagnetic radiation
Chapter 4 Spectroscopy
Chapter 4 Spectroscopy The beautiful visible spectrum of the star Procyon is shown here from red to blue, interrupted by hundreds of dark lines caused by the absorption of light in the hot star s cooler
Temperature and Radiation. What can we learn from light? Temperature, Heat, or Thermal Energy? Kelvin Temperature Scale
What can we learn from light? Temperature Energy Chemical Composition Speed towards or away from us All from the spectrum! Temperature and Radiation Why do different objects give off different forms of
Light, Energy and Matter
Announcements You should see a polling session active if you are using the REEF app. Make sure you are signed in I have thee iclickers to loan out. First come, first served. If you borrow one, please remember