Quiz review

Question 1

What is the energy of a photon
with a wavelength of 500nm? What
is its momentum?

Answer 1

Question 2

Describe polarization and
coherence as properties of light

Question 3

Describe the way in which
light is generated. How do the
generation mechanisms differ?

Answer 3

● Radiation
○ From heat sources
● Radioactivity
○ Gamma rays
● Luminescence
○ Comes from sources other than heat
■ Photoluminescence: results from the absorption of another photon (fluorescence,
phosphorescence, most scattering events)
■ Chemiluminescence: results from a chemical reaction (bioluminescence)
■ Electroluminescence: results from electric current (lasers, LEDs, lightning)
■ Mechanoluminescence: results from mechanical stress (tribo-, piezo-,
sonoluminescence)

Question 4

What is the Stokes Shift?
What light generation method
is it related to?

Answer 4

Stokes shift is the difference (in
energy, wavenumber or frequency
units) between positions of the band
maxima of the absorption and
emission spectra of the same
electronic transition.
Relevant for Fluorescence:
● A larger Stokes shift leads to
fluorescence emission signal
which is easier to distinguish
and measure

Answer 5

● Rayleigh Scattering
○ Elastic scattering on molecules/objects smaller than incident
wavelength; more dependent on wavelength
● Mie Scattering
○ Elastic scattering on molecules/objects similar to or larger than the
incident wavelength; less dependent on wavelength
● Raman Scattering
○ Inelastic scattering of photons by matter, meaning that there is both an
exchange of energy and change in the light’s direction
■ Inelastic: scattered photons have different energy than incident photons

Question 6

Describe two ways in which
biological systems manipulate
light.

Answer 6

Structural colouration in Butterfly Wings:
● Colour is generated by precise constructive and destructive interference
resulting from light reflecting off structures on the surface.
Diatoms as Lenses:
● Diatoms, a type of algae, use structure as a lens to converge light efficiently
for photosynthesis.
Optical Fibers in Sea Sponges (Venus Flower Basket)
● Biological fibers of the sponge conducted light when illuminated and are
stronger and produced via natural processes
Human Eye & Cephalopod colouring as well!

Question 7

What does mean free path
depend on in tissues

Answer 7

Mean free path:
● The average distance over which a moving particle travels before changing
its direction or energy, as a result of collisions with other particles.
● The greater the mean free path → greater penetration → deeper imaging!
The mean free path of a photon travelling through tissue depends on:
● Refractive index variations
● Size of scatterer (mie or rayleigh)
● Wavelength (longer wavelength, greater mean free path)

Question 8

Explain why ion channels
can be modelled as resistors in
the Hodgkin-Huxley Model?

Answer 8

In the plasma membrane ion channels act as
variable resistors because they control the flow
of ions. In electrical circuits, resistors control
the flow of current.
When the channels are open, there is very low
resistance of flow.
When the channels are closed, there is a very
high resistance of low.