Lecture 1-2

Question 1

What do Maxwell’s equations describe?

Answer 1

Generation and propagation of light

Question 2

What are the properties of light?

Answer 2

1. Light carries an electric and magnetic field
2. Light has a frequency and wavelength so it carries energy
3. Light has momentum
4. Light is polarized
5. Light has coherence
6. Light can interact with gravity
7. Light can interact with itself (photon-photon interactions)

Question 3

What are the equations involving light?

Answer 3

c = lamda * v
E = hv = hc/lamda
p = E/c

Question 4

What are Maxwell’s equations?

Answer 4

Gauss’ law of electric charges: ▽E = p/ε0
Gauss’ law of magnetism: ▽B = 0
Faraday’s law (of induction): ▽E = -ϕB/ϕt
Ampere’s law: ▽B = u0J + μ0ε0(ϕE/ϕt)

Question 5

What is Gauss’ law of electric charges?

Answer 5

Electric fields are generated by single charges and diverge

Question 6

What is Gauss’ law of magnetism?

Answer 6

Magnetic fields are generated by dipoles and do not originate in a single point

Question 7

What’s Faraday’s law?

Answer 7

A time-varying magnetic field always accompanies a spatially varying electric field and vice versa (theory of induction)

Question 8

What is Ampere’s law?

Answer 8

Magnetic fields are generated either by electric currents and/or by changing electric fields (solenoids = coiled electrical wire which, when current passes through them, create strong uniform magnetic field)

Question 9

How can classical EM theory describe light?

Answer 9

EM is one of the four main forces in nature (gravity, weak force, EM force, strong force)

Self-propagation of light is composite of 2 waves varying in unison, an electric wave + magnetic wave

2 major acting forces in EM theory: electrostatic forces + magnetic forces

Question 10

What is an EM wave?

Answer 10

Consists of rapidly varying electric + magnetic fields which propagate through space

Energy from light can only be imparted to objects in photons

Amount of energy a photon can impart is proportional to its frequency

Question 11

What is lorentz (EM) force

Answer 11

Measures force exerted on charged particle moving through electric + magnetic field

Responsible for binding electrons to nuclei as well as intermolecular interactions

F = qE + qvB

Question 12

What are the 2 main ways light is generated?

Answer 12

Heat source - thermal radiation (with heat being relative, anything over 0 kelvin)

From emission spectrum of translucent matter interacting with specific spectrum of light

Question 13

What are the radiative heat loss equations?

Answer 13

Pnet = Pemit - Pabsorb
Pnet = Aσε(T^4 - T0^4)

P: power
A: area
Sigma: Boltzmann constant
T: temperature (to envir. temp)

Question 14

What are the main ways in which light is generated?

Answer 14

Radiation - from heat sources
Radioactivity - gamma rays
Luminescence - comes from sources other than heat

Question 15

What kinds of luminescence are there?

Answer 15

Photoluminescence = absorption of another photon (fluorescence, phosphorescence, most scattering events)

Chemiluminescence = chemical reaction

Electroluminescence = electric current (lasers, LEDs, lightning)

Mechanoluminescence = mechanical stress (tribo-, piezo-, sonoluminescence)

Question 16

How is light generated (in general)?

Answer 16

Electrons are excited to higher energy level by means other than heating for luminescence (chemical reaction, electric current, etc.)

Light is generated on discrete levels depending on what substance’s electrons are being excited

When the excited photon drops to normal state, a photon is released

Question 17

Explain exponential decay

Answer 17

A quantity is said to decay exponentially if it decreases at a rate proportional to its current value: dN/dt = -lamda*N

Solution of differential equation: N(t) = N0e^(-lamda*t)

N: quantity
t: time
lamda: exponential decay constant
N(t): quantity at time t
N0: initial quantity (t = 0)

Question 18

Explain mean lifetime and half-life

Answer 18

Mean lifetime of quantity = exponential time constant T = time it takes for quantity to be reduced to 1/e of its initial value

T = 1/lamda = 1/decay constant

Half-life = t(1/2) = time it takes for quantity to be reduced to half its initial value

t(1/2) = ln2/lamda = T*ln2

Question 19

What is fluorescence?

Answer 19

Property of compound to absorb light in a certain range of wavelengths and emit a different wavelength of light (usually longer) - Stokes shift

Shorter time of re-emission (nanoseconds)

Variable mean lifetime or half-life

Question 20

What is phosphorescence?

Answer 20

A phosphorescent material does not immediately re-emit absorbed radiation

The transition between excited and ground states in these materials happens very slowly

Light is re-emitted at a lower intensity for up to several hours after original excitation

Question 21

What are the two kinds of elastic scatterings of light?

Answer 21

Rayleigh: elastic scattering on molecules smaller than incident wavelength

Mie: elastic scattering on molecules similar to or larger than incident wavelength

Both = no energy is lost

Question 22

What is the inelastic scattering of light?

Answer 22

Raman: inelastic scattering of photons by matter - both an exchange of energy and change in light’s direction

Inelastic: scattered photons have different energy (usually lower) than incident photons

Many biosensors being developed using this phenomenon

Question 23

How does chemiluminescence work?

Answer 23

Emission of light that occurs from chemical reactions that produces high amounts of energy lost in the form of photons

A + B -> AB* -> Products + Light
AB* = excited state
Light = ground state

Question 24

How does bioluminescence work?

Answer 24

ATP driven + relate to luciferin (responsible for producing light) + enzyme luciferase

Formula:
Luciferin + O2 -> (Luciferase) -> Oxyluciferin + Light

ATP is hydrolyzed by luciferase enzyme to oxidize luciferin (exciting it) and upon compound’s return to ground state, photon is released

Question 25

How does light interact with matter?

Answer 25

Reflection
Absorption
Emission
Scattering