Homework Set 1 From “Seeing the Light”,Chapter1 : Starting from Page 25 P2, P5, P9, P11, P13, P18, P19, P20 Due: Friday, Feb. 6
WHAT IS LIGHT? a beam Light is a beam of particles that travels on a straight line (I. Newton, 1670s) Geometric Optics (good for optician) Approximately true for many purposes a wave Light is a wave that can interfere and go round about (T. Young, 1803)=> Wave Optics Light is a part of the electromagnetic radiation (Maxwell, Hertz, late 19 th century)
particlesphotons Light is made of particles called photons (Einstein, 1905)=> Particle-Wave Duality The light sometime behaves like a wave and sometime like a beam of particles Quantum Electrodynamics (QED) prediction for the magnetic moment of electron.
Harmonic motion Frequency, Amplitude, Phase Resonance (~ frequency, phase difference ) Sine wave and its properties: Wave length, Speed, Phase Polarization: longitudinal & transverse waves
Harmonic Motion (Oscillation) Simple harmonic motion is seen everywhere in daily life Pendulum, spring, etc period The motion repeats itself. The time it takes to repeat is called period: “T” frequency The inverse of period is called frequency: the number of repetitions in a unit time (sec). Symbol: (nu) Hz Hz = 1 osci./sec
amplitude: “A” The size of the oscillation is called the amplitude: “A” Has to do with energy of the motion characteristic frequency: Every system that undergoes harmonic oscillation has a characteristic frequency: In a spring, this has to do with the mass and strength of the spring. In a pendulum, this has to do with the length of the line and the gravitational constant.
Phase A complete oscillation has a phase of 2 degree. A half-oscillation has a phase of =180 degree, etc. phase difference The phase difference between two oscillators refers to the difference in the stages of their oscillations.
Resonance damping harmonic system A damping harmonic system with oscillates continuously if there is an external driving force which also does harmonic motion. pumping energy to the oscillator => pumping energy to the oscillator Resonance! When the external driving frequency ext is nearby , the energy pumping is very efficient, and Amplitude gets larger & larger: Resonance! ( 90 degree lagging in phase) Resonance Frequency
When the driving frequency is much smaller than the resonance frequency, the oscillator simply follows the external motion (In phase, 0 degree apart) When the driving frequency is much larger than the resonance frequency,the oscillator’s motion is oppositely to that of the driving force (out of phase,180 degree apart) Examples of resonance??
What Is A Wave? There are many examples of waves in daily life: Water wave, sound wave, human wave in a stadium, …, but what is a wave? A wave is a propagating disturbance of some equilibrium, quiescent state. A wave needs a medium, like air, water, people in a stadium. The medium consists of individual “particles” which are normally in a “motionless”, equilibrium state.
Disturbance ? Parts of the medium or some “particles” in the medium move away from the equilibrium because of an external force acting on it. In many examples, the motion of the “particles” is simple harmonic. Wave! Particles in the medium is interactive, when one is off equilibrium, it drags it neighbors into harmonic motion as well (with a little time delay:). Then the disturbance of one particle can propagate in the medium => Wave!
Sine wave and its properties When the particle’s motion is harmonic, the medium can support the simplest wave: Sine Wave. Frequency Frequency of a Sine Wave = frequency of every particle’s oscillation frequency. Wave length Wave length the distance from the nearest particle which does the same oscillation.
= frequency х wavelength Amplitude Amplitude of a wave refers to the amplitude of individual oscillation. Polarization Polarization: Longitudinal wave: the motion of particle is in the same direction as the wave propagation. Transverse wave: The motion of the particle is orthogonal to the wave propagation. (many directions possible) The disturbance is propagating out with a speed :