Lecture 2/3/4 Flashcards
How is light generated
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!
Chemiluminescence
the emission of light by a chemical reaction
Electroluminescence
the emission of light by an electric current
Mechanoluminescence
the emission of light by a mechanical stress
Photoluminescence
the emission of light by the absorption of another photon (stimulated emission)
Bioluminescence
sub-type of chemiluminescence, is the emission of light by biological
systems.
What is fluorescence
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.
quantum yield
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
equation fo quantum yield
phi = (number of photons emitted)/(number of photons absorbed)
fluorescence lifetime
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.
Fluorophores
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!
What is phosphorescence?
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.
Scattering
Scattering, in general, is the deflection and/or diffusion of light. There are three main
types of scattering: Rayleigh, Mie, and Raman.
Rayleigh scattering
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!
Mie scattering
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
Raman scattering
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).
rate of radiative decay
A21 = T21 = 1/t21 (t21:life time)
number of molecules in the excited state
N(t) = N(0)e^(A21t) = N(0)e^(-Trad *t)
total rate of decay
Ttot = Trad +Tnrad
Quantum efficiency
QE = (Trad)/(Tnrad + Trad)
radiative decay
relaxation of excited state to the ground state with emission of radiation
rayleigh scatering
elastic scattering on smaller molecules or objects
raman scattering
inelastic scattering
mie scattering
similar to rayleigh scattering, but on comparable molecules or objects
half-life
t1/2 = ln2/入 = tln2
What is the difference
between chemiluminescence,
fluorescence, and
phosphorescence?
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)
What is the main
difference between
bioluminescence and thermal
radiation?
Bioluminescence: generated from a source other than heat (ATP-driven
chemical reaction)
Thermal Radiation: generated by a heat source
What evolutionary pressures
drive bioluminescence?
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.
What are applications of
chemiluminescence?
Applications of Chemiluminescence:
● Glowsticks
● Forensics
● Protein blotting
● Immunoassay
○ Detection and assay of biomolecules in systems such as ELISA
● Pharmacological
● Toxicological testing
● And many more!
What is the photoelectric
effect?
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.
Wave Equation
superposition princile
Huygens-Fresnel Principle
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
Snell’s Law (based on Fermat’s Principle)
Total Internal Reflection Equation
Brewster’s Angle
lenses
lenses
Prisms
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
Gratings
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.
Polarizers
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
Fibers
n2 always < n1 (n2 outside, n1 inside)
How can optical components
be used to measure ballistic
photons through tissue.
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.
