Redefining Light

1. Redefining Light J Pemberton

2. And God Said… Play the following video and answer the questions on the sheet http://www.youtube.com/watch?v=uYz_kQ7UkY8&feature=related

3. What is an EM Wave? • http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35 Answer the questions on your assignment to do with this applet

4. Hertz’s Experiment This experiment is very significant in the study of light. Find out why.

5. Click on the website below and read about Hertz. Outline his first experiment and the experiment in which he found the speed of radio waves • http://chem.ch.huji.ac.il/history/hertz.htm Watch this movie of Hertz’s experiment http://www.yteach.co.uk/page.php/resources/view_all?id=p5_wave_communication_AM_FM_coding_television_telephone_satellite_navigation_system_GPS_space_amplitude_frequency_modulation_t

6. Black Body Radiation Every object that is warmer than its surroundings is emitting heat. Most objects release that heat in frequencies that are too small to be seen by the naked eye. When an object becomes very hot it releases visible light. The idealized version of a hot body thermodynamically is a black body A black body is a perfect absorber and perfect emitter of heat. We consider the case of a hollow object called a cavity…The hot gas in the cavity will emit heat. Suppose we put a small hole in the side of the cavity. The light that comes out of that hole is the black body radiation of interest. (It is independent of the material from which the body is made)

7. Black Body Radiation Scientists were studying this light in _______ and as light was a wave they had a definite idea of how the light coming from the cavity was going to behave. They expected that there would be a small number of long wavelength waves that could exist in the cavity (as standing waves) but that there would be more and more possible waves at shorter wavelengths that could exist in the cavity. They were expecting the following graph. Brightness of Light at each frequency. (ie amount of light with each frequency) frequencies

8. Black Body Radiation Classical Physics Prediction Brightness of Light at each frequency. (ie amount of light with each frequency) (This graph is usually drawn the other way around with high wavelengths to the right. See below) High ultraviolet Frequency Brightness of Light at each wavelength (ie amount of light with each wavelength) Wavelength

9. Black Body Radiation The graph on the last slide was a worry because it predicted huge amounts of light to be emitted at high frequencies. In the Ultraviolet region of the light spectrum (and even higher frequencies) But this light was missing…They were so worried about this missing light that they called this problem the “Ultraviolet catastrophe” Look at this applet to find out how light really behaves from a black body. Then watch the video .. http://www.usask.ca/chemistry/courses/243/links/BBR/BBR.html http://www.yteach.co.uk/page.php/resources/view_all?id=p5_electric_field_magnetic_electromagnetic_wave_speed_light_information_transfer_page_4&from=search

11. Enter Max Planck http://www.yteach.co.uk/page.php/resources/view_all?id=p5_radiation_object_black_body_Stefan_Boltzmann_law_Wien_grey_star_page_3

12. Black Body Radiation Main Ideas • Bodies can absorb heat • Bodies can emit heat • The way bodies emit heat is really weird!!

13. A B Which furnace is the hottest? C

14. The first clue to solve the catastrophe • View this applet. This shows you the work by Maxwell and others calculated how gas molecules would behave at different temperatures. It is based on statistics. It produces curves that looked familiar to Planck. He asked the question. What if light behaved like particles when it is inside a black body? http://jersey.uoregon.edu/Balloon/index.html

15. Light is a particle?!!!! Important Information • Planck believed that light from a black body could be considered to be a particle. Rather than a wave. (He only used the particle idea for black body radiation it was Einstein who said we could extend the concept to all em waves). The important thing to note is that Planck decided that the energy of light was related to the frequency of the light whereas the energy of the particle in the balloon in related to their velocity. • He said that E=hf The energy of the light particle (called photon) is equal to Planck’s constant times the frequency. This is the main formula for this section of the course. • Try the questions in your assignment to use this formula

16. Planck Theorised that … • Radiation from a black body comes in packets…called quanta (we now call them photons) • The higher the frequency of the packet of light the higher the energy of that packet • E=hf h= Planck’s constant

17. The Photoelectric Effect • Now we come to the important final experiment that helped to confirm that light has particle like properties (even though it is still a wave as well). We will do it in class but here is a simulation until then. Choose Run Now. The podcast will take you through the steps. http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect

18. Other parts of Yteach website that you might like to see. http://www.yteach.co.uk/page.php/resources/view_all?id=p5_radiation_object_black_body_Stefan_Boltzmann_law_Wien_grey_star&from=search http://www.yteach.co.uk/page.php/resources/view_all?id=p5_radiation_object_black_body_Stefan_Boltzmann_law_Wien_grey_star http://www.yteach.co.uk/page.php/resources/view_all?id=p5_wave_matter_particle_Broglie_diffraction_electron_Davisson_Germer_Bohr_orbit_microscope_page_0&from=search Particles or waves http://www.yteach.co.uk/page.php/resources/view_all?id=p5_radiation_object_black_body_Stefan_Boltzmann_law_Wien_grey_star_page_3&from=search

19. Revision • Go through chapter 11 and try the questions at the end of the chapter.