Lec 1-2 Flashcards
Electromagnetic radiation is a self-propagation composite of which two fields?
Electric waves, magnetic waves
Explain what Lorentz force is.
Lorentz force is given by F = qE + qv x B
It is the force exerted on a charged particle moving in an electric and magnetic field
State the names and the conclusions of Maxwell’s equations.
Gauss’s law (electric charges): electric fields are
generated by single charges and diverge
Gauss’s law (magnetism): no “magnetic charge”;
magnetic fields are generated by dipoles and not
originate in a single point
Faraday’s law of induction: a spatially varying
electric field is accompanied by a temporary
varying magnetic field
Ampere’s law: magnetic fields can be generated
in two ways: by electric currents, and/or by
changing electric fields
Write the formula that relate light’s frequency, speed, energy, momentum.
c = lamda v
E = hv = hc/lamda
p = E/c
What objects cannot emit thermal radiation?
Object at T = 0K
Write the formula of thermal radiation
P = A sigma epsilon (Tf^4 - Ti^4)
What is the difference between normal emission and stimulated emission?
Normal emission only emits a photon. Stimulated emission must first absorb an incident photon, then emit two photons.
What are CW laser and pulsed lasers? What’s the advantage of the pulsed laser?
CW laser is continuum wave laser. Its power output is constant with respect to time.
Pulsed laser emits photons in pulses. The peak energy of pulsed laser is larger than CW laser and thus produce more energy. Pulsed laser can be used in medical field like LASIK since it does not produce extra debris.
What makes lasers different than normal light sources?
Coherent, monochromatic, directional
What is the difference between bioluminescence and chemiluminescence?
Chemiluminescence needs a chemical reaction to excite the reactant.
Bioluminescence needs ATP and luciferin.
What is the difference between fluorescence and phosphorescence?
Fluorescence glows shorter (in nanoseconds scale)
Phosphorescence glows longer (in hours)
Define stoke shift
Stoke shift is the difference between the incident light wavelength and the emitted light wavelength in the fluorescence
What are the two elastic scatterings and what is the one that is inelastic? Explain “elastic”.
Elastic scatterings: Rayleigh, Mie.
Inelastic scattering: Raman.
Elastic means the energy is conserved. Inelastic means energy is not conserved.
What is the main difference between Rayleigh scattering and Mie scattering?
Rayleigh scatters small particles (smaller than wavelength).
Mie scatters particles in similar size of the wavelength.
Why the sky is blue?
Light are scattered by Rayleigh scattering. Rayleigh scattering is the best with small wavelength, which is blue.
