Acceleration: Sinusoidal E/M field Sinusoidal Electromagnetic Radiation

1.Radiative pressure – too small to be observed in most cases 2.E/M fields can affect charged particles: nucleus and electrons Both fields (E and M) are always present – they ‘feed’ each other But usually only electric field is considered (B=E/c) Effect of E/M Radiation on Matter

Effect of Radiation on a Neutral Atom Main effect: brief electric kick sideways Neutral atom: polarizes Electron is much lighter than nucleus: can model atom as outer electron connected to the rest of the atom by a spring: F=eE Resonance

Radiation and Neutral Atom: Resonance Amplitude of oscillation will depend on how close we are to the natural free-oscillation frequency of the ball- spring system Resonance

E/M radiation waves with frequency ~10 6 Hz has big effect on mobile electrons in the metal of radio antenna: can tune radio to a single frequency E/M radiation with frequency ~ 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 Importance of Resonance

Three types of receptors (cones) in retina which incorporate three different organic molecules which are in resonance with red, green and blue light frequencies (RGB- vision): Response spectra for three types of receptors Max response wavelengths: S – 440 nm ( Hz) M – 540 nm ( Hz) L – 560 nm ( Hz) Color Vision Refers to length of cone

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 E/M Radiation Transmitters How can we produce electromagnetic radiation of a desired frequency?

a=0 Current in an LC Circuit

Current in an LC circuit Period: Frequency: Current in an LC Circuit Q

Initial energy stored in a capacitor: At time t=0: Q=Q 0 At time t= : Q=0 System oscillates: energy is passed back and forth between electric and magnetic fields. Energy in an LC Circuit 1/4 of a period

What is maximum current? At time t=0: At time t= : Energy in an LC Circuit

Energy in LC Circuit (No dissipation in this circuit) As capacitor loses charge, current increases As capacitor gains charge, current decreases Same equation as obtained via considering potential differences

Frequency: Radio receiver: LC Circuit and Resonance

Question (Chap. 22) A.There will be no current in the circuit at any time because of the opposing emf in the inductor. B.The current in the circuit will maximize at time t when the capacitor will have charge Q(t)=0. C.The current in the circuit will maximize at time t when capacitor will have full charge Q(t)=Q 0. D.The current will decay exponentially. Q0Q0 A capacitor C was charged and contains charge +Q 0 and –Q 0 on each of its plates, respectively. It is then connected to an inductor (coil) L. Assuming the ideal case (wires have no resistance) which of the following is true:

AC voltage (~300 MHz) What will happen if distance is increased twice? no light E/M radiation can be polarized along one axis… …and it can be unpolarized: Polarized E/M Radiation

Making polarized lightTurning polarization Polaroid sunglasses and camera filters: reflected light is highly polarized: can block it Considered: using polarized car lights and polarizers-windshields Polarized Light

Question (preparation for exam) Two metal rings lie side-by-side on a table. Current in the left ring runs clockwise and is increasing with time. This induces a current in the right ring. This current runs A)Clockwise B)Counterclockwise when viewed from above