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Question 1 Which of these is NOT a form of electromagnetic radiation? a) gamma rays b) infrared c) sound d) visible light e) radio Question 1 Which of these is NOT a form of electromagnetic radiation? a) gamma rays b) infrared c) sound d) visible light e) radio Sound comes from pressure waves; all others are types of EM radiation of different wavelengths. Week 6 Day 3 Announcements Feedback on Test 1 • Not bad for first test – but too much talking, TALKING = CHEATING • Next time, make sure people around you do not have your color test before you start the test. • Also, students found talking during remaining tests will receive an immediate 0 and be asked to leave. Grades • Test 1 scores will post by Monday • First iClicker scores will post by tomorrow evening Homework: • Cannot do homework without Mastering Astronomy • Homework counts for 22% of your grade • NOT DOING HOMEWORK REDUCES YOUR CHANCES OF A GOOD GRADE IN THIS CLASS iClickers • Two students are registered for the same iCicker • Only 1 student can be registered per iClicker - Please fix this Electromagnetic Radiation (How we get most of our information about the cosmos) Examples of electromagnetic radiation: Light Infrared Ultraviolet Microwaves AM radio FM radio TV signals Cell phone signals X-rays Radiation travels as waves. Waves carry information and energy. Properties of a wave wavelength (l) crest amplitude (A) trough velocity (v) l is a distance, so its units are m, cm, or mm, etc. Also, v = l n Period (T): time between crest (or trough) passages Frequency (n): rate of passage of crests (or troughs), n = (units: Hertz or cycles/sec) 1 T = hn Waves Demo: making waves - wave table Demo: slinky waves Question 2 The distance between successive wave crests defines the ________ of a wave. a) wavelength b) frequency c) period d) amplitude e) energy Question 2 The distance between successive wave crests defines the ________ of a wave. a) wavelength b) frequency c) period d) amplitude e) energy Light can range from short-wavelength gamma rays to longwavelength radio waves. Radiation travels as Electromagnetic waves. That is, waves of electric and magnetic fields traveling together. Examples of objects with magnetic fields: a magnet the Earth Clusters of galaxies Examples of objects with electric fields: Power lines, electric motors, … Protons (+) "charged" particles that make up atoms. Electrons (-) } Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves. The speed of all electromagnetic waves is the speed of light. c = 3 x 10 8 m / s or c = 3 x 10 10 cm / s or c = 3 x 10 5 km / s light takes 8 minutes Earth Sun c= ln or, bigger l means smaller n The Electromagnetic Spectrum 1 nm = 10 -9 m , 1 Angstrom = 10 -10 m c= ln A Spectrum Demo: white light and a prism Refraction of light All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color. Clicker Question: Compared to ultraviolet radiation, infrared radiation has greater: A: energy B: amplitude C: frequency D: wavelength Clicker Question: The energy of a photon is proportional to its: A: period B: amplitude C: frequency D: wavelength Clicker Question: A star much colder than the sun would appear: A: red B: yellow C: blue D: smaller E: larger Rainbows rred orange yellow green blue violet What's happening in the cloud? raindrop 42o 40o Double Rainbows We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light). What does the spectrum of an astronomical object's radiation look like? Many objects (e.g. stars) have roughly a "Black-body" spectrum: • Asymmetric shape Brightness • Broad range of wavelengths or frequencies • Has a peak Frequency also known as the Planck spectrum or Planck curve. Approximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Law cold dust hotter star (Sun) “cool" star very hot stars frequency increases, wavelength decreases Laws Associated with the Black-body Spectrum Wien's Law: lmax energy a 1 T (wavelength at which most energy is radiated is longer for cooler objects) Stefan's Law: Energy radiated per cm2 of area on surface every second a T 4 (T = temperature at surface) 1 cm2 Betelgeuse Rigel Betelgeuse The total energy radiated from entire surface every second is called the luminosity. Thus Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2) For a sphere, area of surface is 4pR2, where R is the sphere's radius. The "Inverse-Square" Law Applies to Radiation Each square gets 1/4 of the light Each square gets 1/9 of the light apparent brightness a 1 D2 D is the distance between source and observer. The Doppler Effect Applies to all kinds of waves, not just radiation. at rest velocity v1 velocity v2 velocity v1 velocity v1 velocity v3 you encounter more wavecrests per second => higher frequency! fewer wavecrests per second => lower frequency! Doppler Effect Demo: buzzer on a moving arm Demo: The Doppler Ball The frequency or wavelength of a wave depends on the relative motion of the source and the observer. Things that waves do 1. Refraction Waves bend when they pass through material of different densities. air water swimming pool prism air glass air 2. Diffraction Waves bend when they go through a narrow gap or around a corner. 3. Interference Waves can interfere with each other