Chapter 5 Electromagnetic Radiation (Light) Radiation is also known as... Light *pure energy...

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Slide 2 Chapter 5 Electromagnetic Radiation (Light) Slide 3 Radiation is also known as... Light *pure energy Electromagnetic Waves *energy-carrying waves emitted by vibrating electrons Photons *particles of light Slide 4 What is light? Light can act either like a wave or like a particle Particles of light are called photons Slide 5 Waves A wave is a pattern of motion that can carry energy without carrying matter along with it Slide 6 Properties of Waves Wavelength is the distance between two wave peaks Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency Slide 7 Wavelength () and Frequency (f) wavelength x frequency = speed of light ~ 300,000,000m/s f=c Slide 8 Light: Electromagnetic Waves A light wave is a vibration of electric and magnetic fields Light interacts with charged particles through these electric and magnetic fields Slide 9 The Electromagnetic Spectrum A range of light waves extending in wavelength from radio waves to gamma rays Slide 10 The Electromagnetic Spectrum Radio Waves Microwaves Infrared Visible Light Ultraviolet X-rays Gamma Rays Raging Martians Invaded Venus (Roy G. Biv ) Using X-rays and Gamma Rays mnemonic Slide 11 What is the electromagnetic spectrum? Slide 12 Thought Question The higher the photon energy a)the longer its wavelength. b)the shorter its wavelength. c)energy is independent of wavelength. Slide 13 Why be a physicist in late 1800s ? One scientist lamented Newton and Maxwell had done it all. Nothing was left to discover. All that remained was to repeat the experiments with greater and greater precision. But there were some curious problems with classical physics Slide 14 Problems of Classical Physics Spectra from atoms, cold and hot Blackbody radiation Photoelectric effect What is the structure of matter ? Slide 15 Problem:The structure of matter Thompson model of matter had negative electrons stuck in positive pudding. Rutherford alpha scattering experiment proved positive nucleus had most of the mass in small area. Positive particles repel other positive particles. NUCLEI SHOULDNT EXIST a new more powerful force is required. Led to planetary model of atoms. Positive nucleus (protons and neutrons) had negative electrons in orbits around them. Slide 16 More on Structure Problem Charged particles have an electric field around them. Motion of a charged particle causes a disturbance in the field, a wave, i.e. radiation Orbiting electrons should radiate energy, spiral into the nucleus. Slide 17 Spectra from atoms:The Problem Positive charges repel other positive charges with inverse square law. Electrons in classical orbits would emit continuous spectra as they spiral into the nucleus. Atoms shouldnt exist. Slide 18 p. 99 Kirchhoff In 1859 long before atomic structure was understood, Gustav Kirchhoff formulated three rules that describe the three types of spectra. Slide 19 Kirchoffs Laws for Spectra Law I: Solids, liquids or dense gases radiate at all wavelengths, a continuous spectrum. Law II : A low density gas excited to emit light will do so at specific wavelengths, a bright line or emission spectra. Law III : If light from a continuous spectrum passes through a cool low density gas a dark line spectra will result. Line spectra are unique for each element ! Slide 20 Slide 21 Kirchoffs Laws for spectra Slide 22 Kirchhoffs 1 st Law: continuous spectra 1 st law: Solids, liquids or dense gases radiate at all wavelengths we now know the light is radiated by thermal radiation which is sometimes called blackbody radiation Slide 23 Properties of Thermal/Blackbody Radiation 1.Hotter objects emit more light at all frequencies per unit area. 2.Hotter objects emit photons with a higher average energy. Slide 24 Blackbody Radiation The Problem: Attempts to explain BB radiation failed. Classical physics leads to an equation that fits for long wavelengths but has a term that results in increasing intensity as the wavelengths get short. This was called the ultraviolet (uv) catastrophe. Slide 25 Wiens Law for blackbody radiators Slide 26 Wiens Law max Slide 27 Star Colors Reddish coolest star Orange-ish Yellowish White Bluish hottest star Slide 28 Slide 29 3000 K 4000 K 5000 K 6000 K Star Temperatures 10,000 K 15,000 K 30,000 K 7000 K Slide 30 Total Energy from BB The area under the curve is the total energy emitted by the blackbody. Stefan-Boltzmann Law: E = T 4, E is the energy emitted per second is a constant 5.67 X 10 -8 J/m 2 sec/K 4 T is the Kelvin (absolute) temperature of the object A is the area the energy is passing through. Slide 31 Emission Spectrum Something first has to excite the atoms. The surface of a star or near by hot stars mayexcite nebula out to great distances. Slide 32 Slide 33 Horsehead Nebula Slide 34 Slide 35 Absorption Spectra A continuous spectrum passing through a cooler gas will remove (absorb) the same lines the hot gas emits. These are called absorption or dark line spectra. Slide 36 Lab Absorption Slide 37 Stellar Atmospheric Absorption Slide 38 Solar Spectra Slide 39 Next Problem: Photoelectric effect Slide 40 Photoelectric effect:Experimental Slide 41 Photoelectric effect Results: KE of the ejected electrons Slide 42 Photoelectric effect: The Problem Classical physics would predict the brighter the light the more energy the emitted electrons would have. Increasing the intensity gives more electrons, but they all have the same kinetic energy. Each metal has a different threshold wavelength. Below this threshold no electrons are emitted regardless of the intensity of the light. The energy depends linearly on the frequency. E= hf = hc/ Slide 43 What can we find out from the light from a distant source? A. Summary so far: 1.Temperature from BB peak: Wiens Law 2. Total energy from the BB temperature: Stefan- Boltzmann Law 3. Composition: Hot gases, emission nebula Cooler gases, stellar atmospheres, nebula All are descriptive only, not understood until Slide 44 Problems resolved Unexplained events are due to either poor experiments or standing on the edge of a great discovery. The Birth of Modern Physics Max Planck (c 1900) explained BB radiation by applying discrete energy levels to the oscillators in a BB cavity, Plancks constant, h. Einstein took Plancks idea a quantum leap forward by saying the energy itself was emitted in packets of specific energy, photons. Energy given by E = hf. Light has particle properties too. [ This won him the Nobel prize]. It also explains spectra as well. Slide 45 Fig. 6-2, p. 94 Atoms are mostly empty Bohr, Schrodinger and others developed quantum mechanics to describe the structure of matter. We still use the idea of orbitals with associated energy levels, but its better to call these probability distributions. Take a chemistry class. If hydrogen nucleus were the size if a grape seed the first electron orbital would be out 225 yards away. Slide 46 Spectra explained,1 Quantum mechanics, uses particle and wave properties to explain structure of atoms. Electrons in energy levels do not emit energy. Only photons with the exact energy to match differences in energy levels interact, going or coming, with atoms. The lowest energy level is called the ground state. Higher levels are called excited states. Slide 47 Energy Level Transitions The only allowed changes in energy are those corresponding to a transition between energy levels AllowedNot Allowed Im Free Slide 48 Photon absorption The excited atom is unstable and within a fraction of a second returns to a lower energy level, reradiating the photon in a random direction. Slide 49 Fig. 6-4, p. 96 Hydrogen Absorption Slide 50 Spectra explained, 2 Slide 51 p. 100 Hydrogen Energy levels Transitions between levels lead to different series of spectra. To n=1 level Lyman series.. High energy we cant see these. To n=2 Balmer series These are what we see To n= 3, Paschen series, in infrared. Slide 52 Fig. 6-3, p. 95 Energy levels for different atoms Increasing atomic number, more protons pulls the energy levels closer and we get more electrons too, Spectra become more complex. Each element has its signature spectrum. Levels have limits on number of electrons in them (Pauli Exclusion Principle). Slide 53 Chemical Fingerprints Each type of atom has a unique spectral fingerprint Slide 54 Spectra,3 Bright line, emission spectra result from electron transitions to lower energy levels. Conservation of energy requires the energy to be carried away. Photons have energy E = E =hf, E is the energy difference between the levels in the atom. Dark line, absorption, spectra result when an electron is hit by a photon with energy matching a transition to a higher level. The electron jumps to a higher state. The energy is later emitted in either a single photon or a series of photons. Emission is in a random direction. This lowers the light intensity in the original direction. Slide 55 Fig. 6-CO, p. 92 Stellar Spectral Slide 56 Solar Spectra Fraunhoffer in early 1800s found 600 dark lines in solar spectra. We now know the source. Slide 57 Doppler Effect The apparent change in wavelength or frequency of a wave when the source, observer, or both is in motion. Slide 58 Everyday Doppler Effect : Sound Figure 6.11: The Doppler effect. (b) The clanging bell on a moving fire truck produces sounds that move outward (black circles). An observer ahead of the truck hears the clangs closer together, while an observer behind the truck hears them farther apart. Slide 59 Doppler Effect: Light Slide 60 Radar True Velocity Tangential Velocity Radial Velocity Slide 61 Slide 62 Fig. 6-11a, p. 106 Optical Doppler Shift Figure 6.11: The Doppler effect. (a) A blue shift appears in the spectrum of a star approaching Earth (top spectrum). A red shift appears in the spectrum of a star moving away from Earth (bottom spectrum). (The Observatories of the Carnegie Institution of Washington Slide 63 Slide 64 Radial Velocity from Doppler Data !!!! = The ratio of the radial velocity to the speed of the wave is equal to the ratio of the shift in the wavelength to the original wavelength. Or V r = c/ Slide 65 What can we find out from the light from a distant source? B. Summary so far: 1. Temperature from BB peak: Wiens Law 2. Total energy from the BB temperature: Stefan- Boltzmann Law 3. Composition 4. Radial velocity Still more to come.. Slide 66 How does light tell us the rotation rate of an object? Different Doppler shifts from different sides of a rotating object spread out its spectral lines Slide 67 Spectrum of a Rotating Object Spectral lines are wider when an object rotates faster Slide 68 Collisions cause shifts Higher pressure higher probability of a collision Slide 69 Higher Temperatures also cause more broadening Slide 70 Thin cool gas in nebula give sharp lines Slide 71 Demonstrations for Lab 1.View light from a single slit or from a light bulb filament. Vary the voltage changing the temperature. Law I 2. Observe light from selected gas discharge tubes. Law II 3. Observe a solar spectrum by sunlight reflected off a cloud Law III Slide 72