2The Nature of LightOur ideas today on the nature of light are inductive. That is, from observations we build a (set of) model(s) which explain the observations in a consistant fashion.We begin by looking at some everyday observations.
3Observation: Reflection We see reflections everywhere.From the bathroom mirror in the morning, to shiny objects to moonlight on the ocean at night.The Rule:The angles in and out are equalNote that all colors reflect at the same angle.
4Observation: Refraction Refraction is the bending of light when it passes between two different media. For example, water and air.Light will bend toward the normal when moving from a less to greater 'index of refraction' (usually denser material)
5Observation: Refraction Different colors refract at different anglesRefract light through lots of raindrops and you get a rainbowAll colors reflect at the same angle
6Observation: ColorMost of us see color. Our vision ranges from violet (4000Å) to deep red (7000Å)One Angstrom (Å) is 1x10-8 cm
7Corpuscular Model (Particle Model) ReflectionRefractionParticleModelColorLight travels through a vacuum
8Particle Model Reflection Refraction Particles "bounce" Particles slow down in denser materialAngle of incidence = Angle of reflection
9Observation: Scattering Why is the sky blue?Why are the clouds white?Why are sunsets red?
10Observation: Scattering Blue light is scattered all about the sky so that where ever we look we see the blue color
12Corpuscular Model (Particle Model) ReflectionRefractionScatteringParticleModelColorLight travels through a vacuum
13Observation: Polarization Certain crystals and minerals show curious behavior under some circumstances. It was noticed that looking at light reflected off of water through these crystals brightened and dimmed as the crystal was rotated in front of the eyeWe are having a problem with our model.How can particles exhibit this kind of behavior?
14Observation:Diffraction Suppose we shine a light through a narrow opening in a screen, such as sunlight coming through an opening in a shade. We expect to see a bright area pretty much in the same shape as the opening itself.Looking at the screenTop View
15DiffractionIntensityIn many cases, it's more useful to show this as a plot of Intensity (brightness) versus positionLet's close down the slit …position
16Single Slit Diffraction As the slit narrows, instead of the band of light simply getting narrower in proportion, it starts forming bands of light –Diffraction FringesThis is also called aDiffraction PatternThe spacing of the fringes depends on the wavelength and the slit width
17Diffraction This is hard to explain using the particle model of light. If it works for a thin slit, what about a pinhole?
18Circular DiffractionPassing light through a small hole, produces this kind of pattern. Note that the central fringe is much, much greater than the outer ones.
19Straight Edge Diffraction Suppose we block half the light with a straight edgeWe expect to see a sharp shadowInstead we see a diffraction pattern …
20Observation: Interference What happens if two waves interact with each other?You can get some pretty complicated ripple patterns if you tap two fingers on the water surface. We can do the same with light if we put two slits near each other:
21InterferenceSlit separation is 4 times the slit width
22The Model Diffraction Reflection Refraction Scattering Particle PolarizationColorLight travels through a vacuumInterference
23The Wave ModelThere is another model that might be used to explain the observations.What if light were a wave instead of a particle?First, what do we mean by a wave?A wave is a disturbance in a medium.Water waves (ripples displacing the water surface)Sound waves (rarifications and compressions in air)
24Wave Properties The amplitude is how "big" the wave gets A wavelength is one repetition of the patternC = fSpeed of Light = (Wavelength)(Frequency)
25The Wave Model Diffraction Reflection Refraction Scattering Wave PolarizationColorLight travels through a vacuumInterference
27The Wave ModelEverything we have observed can be explained using waves instead of particles – except oneIf a wave is a disturbance in a medium, what is the medium in a vacuum?It was well known that light could travel through a vacuum, but it is hard to "ripple".The Aether was invented.This was an incredibly tenuous medium filling all spaceIt supported the high speed of light, but did not put an appreciable drag on the planets passing through it.
28ModelsAt this point, the wave model can explain most of the observations, it can predict the presence of other new observations.Light now appears to be "just another" aspect of Electromagnetic waves.
30Status We are now close to the turn of the century (1900 that is). The wave theory is becoming more entrenched and can explain more and more phenomena.The particle model is very much in disfavor.Equipment and measurements are getting more and more accurate.Maxwell's Laws predict much of electromagnetism and electromagnetic waves --- except for what produces them.
32More ObservationsThere were four more observations and experiments which are very importantThe Doppler EffectThe Photoelectric EffectBlackbody RadiationThe Michelson-Morley Experiment
33The Doppler EffectLight will shift color depending on the speed of the light source.Only motion toward or away from you causes this effect; there is no color shift for 'sideways' movement.Motion toward you shifts the light toward the blue end of the spectrumMotion away from you shifts the light toward the red end of the spectrum
34Consider a stationary source sending out light pulses
35This time the light source moves to the right as it pulses
36The Doppler EffectObserver sees redder light (the wave crests are farther apart)Observer sees bluer light (the wave crests are closer together)
37Blackbody Radiation Think about heating an old cast-iron frying pan. First, you can feel the heat from the panNext, you can see a dull red glowThen it's cherry redThen orange, yellow, whiteFinally it becomes blue-whiteThe color is an indicator of the temperature6000 °K4000 °K3000 °K>30,000 °K
38Blackbody RadiationRelease of an infinite amount of light at short wavelengths was known as the “Ultraviolet Catastrophe”Max Planck postulated in 1900 that light X-rays, and other waves (i.e. energy) can only be emitted or absorbed in discrete amounts which he called quanta (the plural of "quantum", the Latin word for "how much").The energy quantum is related to the frequency of the wave by a new fundamental constant h
39The Photoelectric Effect Under certain circumstances, light falling on a metal releases electronsThe energy of the electrons is linearly proportional to the frequency of the lightThere will be no electrons if the light is below a certain frequencyThe amount of electron flow is proportional to the intensity of the light.
40The Photoelectric Effect Einstein, using Planck’s idea of a quanta, related the energy of a quanta – or photon – to it’s frequency.The bluer the light, the higher the energy and the more capable of ‘knocking electrons’ out of the metal.
41The Michelson-Morley Experiment In the late 1890's, an attempt was made to measure the motion of the Earth through the 'luminiferous aether'An interferometer was designed to detect the slightest difference in the distance light travels between two separate paths.As the Earth moves, one expects the path lengths to change depending on if they are going with or across the flow of the aetherIncoming lightThere is no measurable changeResultant light
42Which Model ? Interference Reflection Diffraction Refraction PhotoelectricColorWaveModelScatteringParticleModelBlackbodyPolarizationDopplerLight travels through a vacuum
43Which Model ? Both! It's called the "Wave-Particle Duality" It is a model - A view of how it might work. There is no reason why there cannot be several equally valid models. We simply choose the one in which predictions are simplest for a given observation.
44Wave-Particle Duality Of course, if waves (light) sometimes acts as if it were a particle (called a photon) then do particles (electrons, neutrons, etc.) sometimes act as if they were waves?YES! Electron microscopes, Electron diffraction are used to probe the very small structures of nature. Electrons diffract, interfere and exhibit wave behavior under the right conditions.