2Radiation is also known as... Lightpure energyElectromagnetic Wavesenergy-carrying waves emitted by vibrating electronsPhotonsparticles of light
3What is light? Light can act either like a wave or like a particle Particles of light are called photons
4WavesA wave is a pattern of motion that can carry energy without carrying matter along with it
5wave speed = wavelength x frequency Properties of WavesWavelength is the distance between two wave peaksFrequency is the number of times per second that a wave vibrates up and downwave speed = wavelength x frequency
6Wavelength (λ) and Frequency (f) wavelength x frequency = speed of light ~ 300,000,000m/s λf=c
7Light: Electromagnetic Waves A light wave is a vibration of electric and magnetic fieldsLight interacts with charged particles through these electric and magnetic fields
8The Electromagnetic Spectrum A range of light waves extending in wavelength from radio waves to gamma rays
9The Electromagnetic Spectrum Radio WavesMicrowavesInfraredVisible LightUltravioletX-raysGamma RaysRagingMartiansInvadedVenus (Roy G. Biv )UsingX-rays andGamma Raysmnemonic
11Thought Question The higher the photon energy… the longer its wavelength.the shorter its wavelength.energy is independent of wavelength.
12Why 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”
13Problems of Classical Physics Spectra from atoms, cold and hotBlackbody radiationPhotoelectric effectWhat is the structure of matter ?
14Problem: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 SHOULDN’T 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.
15More 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. radiationOrbiting electrons should radiate energy, spiral into the nucleus.
16Spectra 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 shouldn’t exist.
17KirchhoffIn 1859 long before atomic structure was understood, Gustav Kirchhoff formulated three rules that describe the three types of spectra.p. 99
18Kirchoff’s 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 !
21Kirchhoff’s 1st Law: continuous spectra 1st 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
22Properties of Thermal/Blackbody Radiation Hotter objects emit more light at all frequencies per unit area.Hotter objects emit photons with a higher average energy.Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.
23Blackbody RadiationThe 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.Figure 6.6: Black body radiation from three bodies at different temperatures demonstrates that a hot body radiates more total energy and that the wavelength of maximum intensity is shorter for hotter objects. The hotter object here will look blue to our eyes, whereas the cooler object will look red.
25Wien’s LawFigure 6.6: Black body radiation from three bodies at different temperatures demonstrates that a hot body radiates more total energy and that the wavelength of maximum intensity is shorter for hotter objects. The hotter object here will look blue to our eyes, whereas the cooler object will look red.l max =
26Star Colors Reddish coolest star Orange-ish Yellowish White Bluish hottest star
28Star Temperatures 3000 K 4000 K 5000 K 6000 K 7000 K 10,000 K 15,000 K
29Total Energy from BBThe area under the curve is the total energy emitted by the blackbody.Stefan-Boltzmann Law:E = sAT4,E is the energy emitted per secondσ is a constant 5.67 X 10-8 J/m2sec/K4T is the Kelvin (absolute) temperature of the objectA is the area the energy is passing through.
30Emission SpectrumSomething first has to excite the atoms. The surface of a star or near by hot stars mayexcite nebula out to great distances.
40Photoelectric effect Results: KE of the ejected electrons
41Photoelectric 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/λ
42What can we find out from the light from a distant source? A. Summary so far:Temperature from BB peak: Wien’s Law2. Total energy from the BB temperature: Stefan-Boltzmann Law3. Composition: Hot gases, emission nebulaCooler gases, stellar atmospheres, nebulaAll are descriptive only, not understood until …
43Problems resolvedUnexplained events are due to either poor experiments or standing on the edge of a great discovery.The Birth of Modern PhysicsMax Planck (c 1900) explained BB radiation by applying discrete energy levels to the oscillators in a BB cavity, Planck’s constant, h.Einstein took Planck’s 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.
44Atoms are mostly emptyBohr , Schrodinger and others developed “quantum mechanics” to describe the structure of matter.We still use the idea of orbitals with associated energy levels, but it’s better to call these probability distributions.Take a chemistry class.Figure 6.2: Magnifying a hydrogen atom by 1012 makes the nucleus the size of a grape seed and the diameter of the electron cloud about 4.5 times longer than a football field. The electron itself is still too small to see.If hydrogen nucleus were the size if a grape seed the first electron orbital would be out 225 yards away.Fig. 6-2, p. 94
45Spectra explained,1Quantum 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.
46Energy Level Transitions I’m FreeThe only allowed changes in energy are those corresponding to a transition between energy levelsNot AllowedAllowed
47Photon absorptionFigure 6.5: An atom can absorb a photon only if the photon has the correct amount of energy. 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.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.
48Hydrogen Absorption Fig. 6-4, p. 96 Figure 6.4: A hydrogen atom can absorb only those photons that move the atom’s electron to one of the higher-energy orbits. Here three different photons are shown along with the change they would produce if they were absorbed.Fig. 6-4, p. 96
50Hydrogen Energy levels Transitions between levels lead to different series of spectra.To n=1 level Lyman series .. High energy we can’t see these.To n=2 Balmer series… These are what we seeTo n= 3 , Paschen series, in infrared.p. 100
51Energy levels for different atoms Increasing atomic number, more protons pulls the energy levels closer and we get more electrons too,Spectra become more complex.Figure 6.3: The electrons in atoms may occupy only certain, permitted orbits. Because different elements have different charges on their nuclei, the elements have different patterns of permitted orbits.Each element has its signature spectrum. Levels have limits on number of electrons in them (Pauli Exclusion Principle).Fig. 6-3, p. 95
52Chemical Fingerprints Each type of atom has a unique spectral fingerprint
53Spectra,3Bright 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 = DE =hf,DE 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.
55Solar SpectraFraunhoffer in early 1800’s found 600 dark lines in solar spectra. We now know the source.
56Doppler EffectThe apparent change in wavelength or frequency of a wave when the source, observer, or both is in motion.
57Everyday 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.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.
58Doppler Effect: LightFigure 6.11: The Doppler effect. (c) A moving source of light emits waves that move outward (black circles). An observer in front of the light source observes a shorter wavelength (a blue shift), and an observer behind the light source observes a longer wavelength (a red shift).
61Optical Doppler ShiftFigure 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)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 WashingtonFig. 6-11a, p. 106
63Radial 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. OrVr = Δλ c/ λ
64What can we find out from the light from a distant source? B. Summary so far:1. Temperature from BB peak: Wien’s Law2. Total energy from the BB temperature: Stefan-Boltzmann Law3. Composition4. Radial velocityStill more to come…..
65How does light tell us the rotation rate of an object? Different Doppler shifts from different sides of a rotating object spread out its spectral linesThis figure from the book can give an introduction to the Doppler effect.
66Spectrum of a Rotating Object This figure from the book can give an introduction to the Doppler effect.Spectral lines are wider when an object rotates faster
67Collisions cause shifts Higher pressure higher probability of a collision
70Demonstrations for Lab 1.View light from a single slit or from a light bulb filament.Vary the voltage changing the temperature. Law I2. Observe light from selected gas discharge tubes. Law II3. Observe a solar spectrum by sunlight reflected off a cloudLaw III