Lecture 2/3/4

Question 1

How is light generated

Answer 1

Light can be emitted by heat sources, such as through fusion and other chemical reactions.
◆ Light can be generated through luminescence. This is expanded upon below.
◆ Light can also be emitted through radioactivity.
◆ Non-radiative decay involves the release of energy through heat. In other words, instead
of the energy being released in the form of a photon, the material simply heats up!

Question 2

Chemiluminescence

Answer 2

the emission of light by a chemical reaction

Question 3

Electroluminescence

Answer 3

the emission of light by an electric current

Question 4

Mechanoluminescence

Answer 4

the emission of light by a mechanical stress

Question 5

Photoluminescence

Answer 5

the emission of light by the absorption of another photon (stimulated emission)

Question 6

Bioluminescence

Answer 6

sub-type of chemiluminescence, is the emission of light by biological
systems.

Question 7

What is fluorescence

Answer 7

Fluorescence is the absorption of light, quickly followed by its emission.
* One property of compounds is the ability to absorb light within a particular
wavelength range, and to then emit light at a different wavelength.
* The difference between absorption wavelength and emission wavelength is
referred to as the Stokes shift.

Question 8

quantum yield

Answer 8

The quantum yield of a compound is the ratio of the number of emitted photons over the
number of absorbed photons. This value will always be less than one, as some energy is
11
always lost to heat

Question 9

equation fo quantum yield

Answer 9

phi = (number of photons emitted)/(number of photons absorbed)

Question 10

fluorescence lifetime

Answer 10

The fluorescence lifetime is the measure of how long the molecule in question will
remain in the excited state. In other words, it is the amount of time the light emission
will last. This is normally measured in nanoseconds.

Question 11

Fluorophores

Answer 11

Fluorophores are molecules that fluoresce.
* Endogenous fluorophores are naturally occurring within cells.
* Exogenous fluorophores are introduced to cells, usually for experimental
purposes. In fact, jellyfish-derived Green Fluorescent Protein (GFP) is often used in
laboratory settings to tag other proteins, allowing for simplified visualization!

Question 12

What is phosphorescence?

Answer 12

Much like fluorescence, phosphorescence involves the absorption and subsequent
emission of light. However, the emission takes far longer.
◆ While fluorescence lifetimes are on the order of nanoseconds, phosphorescence lifetimes
are on the order of hours.
◆ Glow-sticks use phosphorescence to be able to glow for hours!
5.

Question 13

Scattering

Answer 13

Scattering, in general, is the deflection and/or diffusion of light. There are three main
types of scattering: Rayleigh, Mie, and Raman.

Question 14

Rayleigh scattering

Answer 14

Rayleigh scattering is elastic scattering on objects smaller than the wavelength of light.
Elastic scattering means that the interaction between the object and the light does not
involve energy transfer. A popular example is the answer to the question “why is the
sky blue?”: atmospheric particles scatter the shortest wavelengths of light best, which
corresponds to blue light with regards to the visible spectrum. This is a case of Rayleigh
scattering as the particles are smaller than the wavelength of light being reflected!

Question 15

Mie scattering

Answer 15

Mie scattering is similar to Rayleigh scattering in that it is elastic, but it applies to objects
similar in size the light’s wavelength instead. In both cases, the following formula and
diagram apply:
x=(2pi*r)/入

Question 16

Raman scattering

Answer 16

Raman scattering is inelastic scattering, meaning an energy exchange occurs upon contact
between the light and the object. This results in a polarizability change in the molecules
struck by the light. As shown in the diagram below, the energy of the scattered photon
does not match that of the incident photon! This contrasts with Rayleigh and Mie
scattering (see the diagram above).

Question 17

rate of radiative decay

Answer 17

A21 = T21 = 1/t21 (t21:life time)

Question 18

number of molecules in the excited state

Answer 18

N(t) = N(0)e^(A21t) = N(0)e^(-Trad *t)

Question 19

total rate of decay

Answer 19

Ttot = Trad +Tnrad

Question 20

Quantum efficiency

Answer 20

QE = (Trad)/(Tnrad + Trad)

Question 21

radiative decay

Answer 21

relaxation of excited state to the ground state with emission of radiation

Question 22

rayleigh scatering

Answer 22

elastic scattering on smaller molecules or objects

Question 23

raman scattering

Answer 23

inelastic scattering

Question 24

mie scattering

Answer 24

similar to rayleigh scattering, but on comparable molecules or objects

Question 25

half-life

Answer 25

t1/2 = ln2/入 = tln2

Question 26

What is the difference between chemiluminescence, fluorescence, and phosphorescence?

Answer 26

Fluorescence and Phosphorescence are based on ability of substance to absorb light and emit light of a longer wavelength and lower energy. The difference between these two is that phosphorescence lingers for a longer period of time. Chemiluminescence needs activation for chemical reaction (requires ATP in the bioluminescent case)

Question 27

What is the main difference between bioluminescence and thermal radiation?

Answer 27

Bioluminescence: generated from a source other than heat (ATP-driven chemical reaction) Thermal Radiation: generated by a heat source

Question 28

What evolutionary pressures drive bioluminescence?

Answer 28

Two Main Evolutionary Pressures: 1. Survival a. Counterillumination camouflage, mimicry of other animals, for example to lure prey or escape predators. 2. Reproduction a. Signaling to other individuals of the same species to attract mates.

Question 29

What are applications of chemiluminescence?

Answer 29

Applications of Chemiluminescence: ● Glowsticks ● Forensics ● Protein blotting ● Immunoassay ○ Detection and assay of biomolecules in systems such as ELISA ● Pharmacological ● Toxicological testing ● And many more!

Question 30

What is the photoelectric effect?

Answer 30

Photoelectric Effect: The emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. Photoemission can occur from any material, but it is most common in metals and other conductors.

Question 31

Wave Equation

Answer 31

Question 32

superposition princile

Answer 32

Question 33

Huygens-Fresnel Principle

Answer 33

When light interacts with a material at an interface, points of interaction will be sources for new spherical waves. ● These waves will then interact with each other, creating a wave front

Question 34

Snell’s Law (based on Fermat’s Principle)

Answer 34

Question 35

Total Internal Reflection Equation

Answer 35

Question 36

Brewster’s Angle

Answer 36

Question 37

lenses

Answer 37

Question 37

lenses

Answer 37

Question 38

Prisms

Answer 38

A prism works because the different colors of light travel at different speeds inside the glass. They get bent by different amounts and spread out as they leave the prism. ● Used in spectrometers to analyze spectral components of light

Question 39

Gratings

Answer 39

A diffraction grating is an optical element that disperses light composed of lots of different wavelengths(e.g., white light) into light components by wavelength. ● When white light enters the grating, the light components are diffracted at angles that are determined by the respective wavelengths.

Question 40

Polarizers

Answer 40

Def. Polarization: a property that applies to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. In linear polarization, the fields oscillate in a single direction. ● Think of the polarizer as a vertical fence, and the electronic wave of light as needing to move through it. ● The only waves that are able to pass through the fence slits, all other polarizations are blocked

Question 41

Fibers

Answer 41

n2 always < n1 (n2 outside, n1 inside)

Question 42

How can optical components be used to measure ballistic photons through tissue.

Answer 42

Ballistic photons can be detected using coherent high resolution medical imaging systems. An example is optical coherence tomography (OCT). OCT uses mirrors, lenses, beam splitters, and optical fibers and filters to measure the amount of ballistic photons passed through a tissue, and create a medical image. Imaging using ballistic photons higher resolution, but suffers from limited imaging depth.