Re-radiation: Scattering lectric fields are not blocked by matter: how can decrease?
Cardboard Why there is no light going through a cardboard? lectric fields are not blocked by matter lectrons and nucleus in cardboard reradiate light Behind the cardboard reradiated /M field cancels original field
ffect of /M Radiation on Matter 1. Radiative pressure too small to be observed in most cases 2. /M fields can affect charged particles: nucleus and electrons Both fields ( and M) are always present they feed each other But usually only electric field is considered (B=/c)
ffect of Radiation on a Neutral Atom Main effect: brief electric kick sideways Neutral atom: polarizes lectron is much lighter than nucleus: can model atom as outer electron connected to the rest of the atom by a spring: Resonance F=e See 16.P.67
Radiation and Neutral Atom: Resonance y F y = = e sin y ( ωt) = F sin ( ωt) Amplitude of oscillation will depend on how close we are to the natural free-oscillation frequency of the ballspring system Resonance
Importance of Resonance /M radiation waves with frequency ~1 6 Hz has big effect on mobile electrons in the metal of radio antenna: can tune radio to a single frequency /M radiation with frequency ~ 1 15 Hz has big effect on organic molecules: retina in your eye responds to visible light but not radio waves Very high frequency (X-rays) has little effect on atoms and can pass through matter (your body): X-ray imaging
lectromagnetic Spectrum
Color Vision Three types of receptors (cones) in retina which incorporate three different organic molecules which are in resonance with red, green and blue light frequencies (RGBvision): Response spectra for three types of receptors Max response wavelengths: S 44 nm (6.81. 1 14 Hz) M 54 nm (5.56. 1 14 Hz) L 56 nm (5.36. 1 14 Hz) Refers to length of cone
/M Radiation Transmitters How can we produce electromagnetic radiation of a desired frequency? Need to create oscillating motion of electrons Radio frequency LC circuit: can produce oscillating motion of charges To increase effect: connect to antenna Visible light Heat up atoms, atomic vibration can reach visible frequency range Transitions of electrons between different quantized levels
Polarized /M Radiation AC voltage (~3 MHz) What will happen if distance is increased twice? no light /M radiation can be polarized along one axis and it can be unpolarized:
Polarized Light Making polarized light Turning polarization Polaroid sunglasses and camera filters: reflected light is highly polarized: can block it Considered: using polarized car lights and polarizers-windshields
Why the Sky is Blue Why there is light coming from the sky? Why is it polarized? Why is it blue? ( ) x = x sin ωt ~ a = d 2 x dt 2 nergy flux: = y ω 2 sin( ωt) 2 4 ~ ~ ω Ratio of blue/red frequency is ~2 scattering intensity ratio is 16 Why is sun red at sunset? Is its light polarized? Why are distant mountains blue?
Chapter 25 Waves and Particles
Particles or Waves? Isaac Newton (1643 1727) Christian Huygens (1629 1695)
Wave Phenomena Interference Diffraction Reflection
Wave Description λ wavelength: distance between crests (meters) T period: the time between crests passing fixed location (seconds) v speed: the distance one crest moves in a second (m/s) f frequency: the number of crests passing fixed location in one second (1/s or Hz) ω angular frequency: 2πf: (rad/s) v = λ T f 1 = v = λf T
Wave: Variation in Time = cos ωt ( ) = cos 2π T t
Wave: Variation in Space 2πx 2π = cos = cos x λ λ
Wave: Variation in Time and Space 2π = cos t T 2π = cos x λ 2π 2π = cos t x T λ - sign: the point on wave moves to the right
Wave: Phase Shift 2π 2π = cos t x T λ But @ t= and x =, may not equal 2π 2π = cos t x + φ T λ 2π = cos t + φ = cos ωt T Two waves are out of phase (Shown for x=) phase shift, φ= 2π ( + φ)
Wave: Amplitude and Intensity = cos t ( ω +φ) is a parameter called amplitude (positive). Time dependence is in cosine function Often we detect intensity, or energy flux ~ 2. For example: Vision we don t see individual oscillations Intensity I (W/m 2 ): 2 I Works also for other waves, such as sound or water waves.
Interference Superposition principle: The net electric field at any location is vector sum of the electric fields contributed by all sources. Can particle model explain the pattern? Laser: source of radiation which has the same frequency (monochromatic) and phase (coherent) across the beam. Two slits are sources of two waves with the same phase and frequency.
Interference: Constructive 1 Two emitters: 2 Fields in crossing point 1 2 = = cos cos ( ωt) ( ωt) Superposition: = + cos( ωt) 1 2 = 2 Amplitude increases twice: constructive interference
Interference: nergy 1 Two emitters: 2 = 1 + 2 = 2 cos ( ωt) What about the intensity (energy flux)? nergy flux increases 4 times while two emitters produce only twice more energy There must be an area in space where intensity is smaller than that produced by one emitter
Interference: Destructive 2 1 1 2 = = cos cos ( ωt) ( ωt + π ) ( cos( ω t) + cos( ω + π ) = + = t 1 2 = cos( ωt) Two waves are 18 out of phase: destructive interference
Interference Superposition principle: The net electric field at any location is the vector sum of the electric fields contributed by all sources. Constructive: 1 2 = = cos cos = 1 + 2 = ( ωt) ( ωt) 2 cos ( ωt) ( cos( ω t) + cos( ω + π ) = + = t 1 2 1 Destructive: 2 = = cos cos ( ωt) ( ωt + π ) Amplitude increases twice Two waves are 18 out of phase Constructive: nergy flux increases 4 times while two emitters produce only twice more energy
Interference Intensity at each location depends on phase shift between two waves, energy flux is redistributed. Maxima with twice the amplitude occur when phase shift between two waves is, 2π, 4π, 6π (Or path difference is, λ, 2 λ ) Minima with zero amplitude occur when phase shift between two waves is π, 3π, 5π (Or path difference is, λ/2, 3λ/2 ) Can we observe complete destructive interference if ω 1 ω 2?