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Redefining Light

J Pemberton

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And God Said…

http://www.youtube.com/watch?v=uYz_kQ7UkY8&feature=related

Play the following video and answer the questions on the sheet

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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

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Hertz’s Experiment

This experiment is very significant in the study of light. Find out why.

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• 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

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

Watch this movie of Hertz’s experiment

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Black Body RadiationEvery 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)

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Black Body RadiationScientists 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.

frequencies

Brightness of Light at each frequency. (ie amount of light with each frequency)

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Black Body Radiation Classical Physics Prediction

Frequency

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)

Wavelength

Brightness of Light at each wavelength (ie amount of light with each wavelength)

High ultraviolet

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Black Body RadiationThe 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

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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

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Black Body Radiation

Main Ideas• Bodies can absorb

heat• Bodies can emit

heat• The way bodies

emit heat is really weird!!

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Which furnace is the hottest?

A

B

C

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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

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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

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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

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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

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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=searchParticles or waves

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_page_3&from=search

http://www.yteach.co.uk/page.php/resources/view_all?id=p5_radiation_object_black_body_Stefan_Boltzmann_law_Wien_grey_star

Other parts of Yteach website that you might like to see.

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Revision

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