Lab topics Flashcards
Define absorbance
How much light is absorbed by medium after passing through
A = log J0/J
Define transmittance
How much light is left after passing through a medium
T = J/J0 100
How much light is transmitted by a sample with an absorbance of 1?
A = log J0/J
1 = log J0/J1
J0/J1 = 10
T = J/J0 * 100 = 1/10*100 = 10%
Which sample transmits more light: OD=1 or OD=3? By how much?
OD = 1 transmits one hundred-fold more light
OD = 1 = log J0/J
T = J0/J = 1/10 = 0.1
OD = 3 = log J0/J
T = J0/J = 1/1000 = 1*10^-3
How does the absorption spectrum change if the sample concentration is doubled?
It doubles (due to the linear relationship in Beer Lambert’s law)
log (J0/J) = Ɛ (λ) c x
How does the absorption spectrum change if the sample concentration is halved?
It halves (due to linear relationship in Beer Lambert’s law)
What is the absorption maximum characteristic of?
- The material’s molecular structure (electron excitation energies of that material).
- We use it to measure the concentration of a material in a solution.
What is the function of the monochromator?
- used to isolate and select a narrow range of wavelengths from a broader spectrum of light.
- separate light into its components, allowing only a specific wavelength or a narrow band of wavelengths to pass through.
Define Biot-law
- The angle of rotation can be calculated using the specific angle of rotation (depending on the type of sugar), the concentration, and the pathlength (dm) of the solution
a = [a]20d *c * l
Describe the linearly polarized light.
An electromagnetic wave, in which the electric field line vector propagates in one plane only.
What light source is used for polarimetry and why?
Monochromatic light is used because the specific angle constant is unique to a certain type of wavelength
the optical rotation angle and concentration
directly proportional
What is a chiral molecule? Provide an example
- A molecule that has a non-superimposable mirror image with four different ligands
- e.g., D-glucose and L-glucose
Factors influencing specific optical rotation
- Wavelength (D= 589nm for sodium lamp)
- Temp (20 deg)
How do you determine concentration by polarimetry?
Biot’s law: knowing the angle of rotation and the length of the tube (dm)
Refractive media of the eye. Image formation by the eye
- Air - Cornea - aqueous humor - lens - vitreous humor
- Light rays converge on the center of retina (fovea centralis)
What is the refractive power of the unaccommodated human eye?
- 64 dioptres
- Cornea contributes 43D and lens contribute 23D
Which refractive surface contributes the most to the refractive power of the human eye?
Cornea (43D) because of big difference in n of air (around 1.00) and cornea
Describe the circularly polarized light.
- Composed of two perpendicular linearly polarized light waves with matching wavelength and amplitude
- The electric field vector of the electromagnetic wave rotates in a circular motion as the wave propagates through space
Describe the process of focal accommodation of the human eye
- Ciliary muscles contract
- suspensory ligament relaxes
- lens becomes more budged
- Radius decreases
- Power increases → allows you to see closer object
How do you calculate the accomodation power of human eye?
D = 1/op - 1/or
Op- Nearest d of clear vision
Or = farthest d of clear vision
How would you measure the position and diameter of the blind spot?
Diameter (mm)=
(1760)/d where spot reappears -
(1760)/d spot where disappears
Position (mm) =
[(1760)/d where spot reappears +
(1760)/d spot where disappears]/2
Position is relative to distance of center of blind spot to fovea centralis
What is myopia and how do you correct it?
- Nearsightedness, elongated eyeball, image focused behind retina
- Can see close but not far
- Divergent lens needed (concave)
What is hyperopia and how do you correct it?
- Farsightedness, shortened eyeball, image
- formed behind retina
- Can see far but not close
- Convergent lens needed (convex)
How does the refractive power of the human eye change during accommodation?
Refractive power (D) of human eye increases
- because radius of the lens decreases
What is visual acuity and how do you measure it?
1 min/limiting angle in arc min = visual acuity in visus
Visual acuity: how sharp eyesight is (ability to detail image)
How did we measure visual acuity?
Using Landolt’s broken ring where
Limiting angle (rad) = size of break/ distance to ring
Describe the reduced eye model
- idealized model of the optics of the human eye introduced by Franciscus Donders
- n=1.34
- Curvature of refractive surface = 5.1mm
- nodal point (K) is 17mm from yellow spot
Factors influencing visual acuity.
- Irregular shape of the lens and the eyeball
- Diffraction (causing airy disks)
- Anatomical density of photoreceptors (rods and cons)
Spatial distribution of photoreceptors on the retina
- Cones are tightly packed (like honeycomb) with around 2 micrometer distance between
- Cones are most densely packed in the fovea centralis (no rods), visual acuity is highest there
- Rods are located in the periphery of the eye, where the visual acuity is lowest.
How do you determine the spring constant of a cantilever?
- hooke’s law: F = -kx
- plot displacement vs force
- slope is spring constant
What is the visual acuity of a patient with a limiting angle of vision of 2’
½ * 100 = 50%
Parts of the scintillation counter
Scintillator: A scintillator produces photons or flashes of light as a particle passes
Photodetector: photomultiplier tube, converts the photon from the scintillator into an electrical signal that a digital counter can then read
Sources of noise in the scintillation counter
- external noise (ie. background radiation)
- internal noise (ie. from electronics):
higher V = more noise
How can external noise be reduced in scintillation counting?
lead shielding (place isotope in a lead holder with a narrow slit called a collimator serves this purpose)
How can internal noise be reduced in scintillation counting?
Using the minimum required voltage of the photomultiplier
Describe the function of the integral discriminator
- Signal-selecting device that ignores pulses below a certain amplitude
- filter out noise improving the signal-to-noise ratio.
Define the signal-to-noise ratio
Ratio of useful signal and useless noise
higher value = more accurate reading
Signal to noise ratio = number of signals/ number of noise pulses.
How do you find the optimal integral discriminator setting of the scintillation counter?
Highest signal to noise ratio, voltage already set
How many electrons arrive at the PMT anode for every photoelectron if the number of the dynodes is 8 and the multiplication factor is 2. Why?
2^8 -> 256 electrons
Exponential because every collision with the dynode produces a secondary electron (double at every collision)
What is presbyopia and how do you correct it?
- ability to see things close-up gets worse with age due to loss of lens elasticity
- Convergent reading glasses to correct
Define surface density
the amount of mass per cm^2
Unit is g/cm^2
Define the attenuation coefficient
how easily a volume of material can be penetrated by wave or matter
does depend on density or material, just intensity of radiation
Unit: 1/m
Define the half-value layer thickness
the thickness of the material when intensity of radiation entering is reduced by one half
Define the tenth-value layer thickness.
the thickness of the material when intensity of radiation entering is reduced by one tenth
Explain the energy dependence of mass attenuation coefficient in case of lead
At lower energies, photoelectric absorption dominates, leading to a higher mass attenuation coefficient
As energy increases, Compton scattering becomes more significant, causing a decrease in the coefficient
pair production becomes relevant at very high energies (above 1eV)
Compare the linear and mass attenuation coefficients of liquid water and steam
Both the linear and mass attenuation coefficients are higher for liquid water compared to steam due to the higher density of liquid water
Liquid water stronger attenuation
What fraction of intensity is transmitted through an absorber with a thickness twice its half-value layer thickness (x=2D)
J/J0 = e^-μx
J/J0 = e^(-ln 2)(2) = 1/4
What fraction of intensity is transmitted through an absorber with a thickness three times its half-value layer thickness (x=3D).
J/J0 = e^(-ln 2)(3) = ⅛
Harmonic oscillation (definition, equation, graph)
Definition: when restoring force is directly proportional to displacement
Equation: x = A sin (ωt)
A: amp, ω: angular frequency
Graph: sinusoidal wave, displacement-time
Damped free oscillation
- amp decreases exponentially over time
- Realistic oscillation where there is a loss of energy due to friction
Driven oscillation, resonance
- damped oscillators further affected by an externally applied force F(t)
- Resonance occurs when the frequency of the driving force is close to the eigenfrequency of the system
- results in big amplitude.
Resonance curve
- shows amp-freq function
- highest amp = eigenfrequency
x-axis: frequency (f)
y-axis: amplitude (A)
if the oscillatory mass is doubled
resonance frequency decreases by sqrt(1/2)
if spring constant is doubled
resonance frequency would increase by sqrt(2)
Define the eigenfrequency
- natural frequency/resonance frequency
- system oscillates without any external force
f = 1/2pi sqrt(k/m)
Define the mass attenuation coefficient
howmuch wave is weakened by the material it is passing through
unit is cm2/g
Gain and gain level of the amplifier
gain: how loud
- Power gain (Ap) = output/input power
- Voltage gain (Au) = output/input voltage
gain level: log scale of gain (dB)
Compare voltage gain and power gain.
Power gain
Ap = 10 log (P out/ P in)
Voltage gain
Au = 20 log (V out / V in)
What is the gain level if the voltage gain is 1000?
20log(Au) -> 20log(1000) = 60dB
What is the gain level if the voltage gain is 1?
20log(Au) -> 20log(1) = 0
What is the power gain if the gain level is 3dB?
3db = 10log(Ap) -> 10^(3/10) -> 2
Frequency response curve of the amplifier.
describes the dependence of gain level on the frequency
x-axis: log scale of f (Hz)
y-axis: power gain level (dB)
How do you determine the transfer band of an amplifier
plot two point
point 1: smallest x, nmax-3
point 2: biggest x, nmax-3
this gives you the upper and lower cut-off frequencies
How does the bandwidth of an amplifier change with negative feedback
increase in transfer bandwidth (wider frequency)
Advantages and disadvantages of using negative feedback in an amplifier.
advantage: stable, less distortion, wide transfer band
disadvantage: decreased gain, need more amplification
Voltage divider circuit
- a simple series resistor circuit
- Its output voltage is a fixed fraction of its input voltage determined by two resistors
Radius of curvature (R) and refractive power (D)
- inversely proportional
- This is because D is determined by focal length which is affected by R
D = 1/f
radius of curvature in the case of a lens
distance from the vertex to the center of curvature
index of refraction (n) and refractive power (D)
- directly proportional
- n: indicates light bending quality of medium
- Lensmaker equation:
D = 1/f = (n-1)(1/r1 + 1/r2)
refractive power
D = 1/f
within the focal distance of a converging lens
- on same side
- Magnified, virtual, upright
between the single and the double focal distance of a converging lens
- beyond 2f
- magnified, real, inverted
outside the double focal distance of a converging lens
- between F2 and 2F2
- diminished, real, inverted
image formed by a compound light microscope
Magnified, inverted, virtual
Objective: magnifies on real image plane (intermediate image)
Ocular lens: magnifies to produce virtual image
the total magnification of a light microscope if the objective magnification is 100x and the ocular magnification is 20x?
100*20 = 2000x
Describe the steps of the eyepiece scale calibration process
Eyepiece + stage micrometer
- Align zero’s on both tools
- Find second intersection where line perfectly aligns
- note the eyepiece unit and stage in which line intersects
- Stage/eyepiece = 1 eyepiece equals how many stage micrometers
prisms are present in the Abbe-refractometer?
- Illuminating prism (rough surface)
- measuring prism (flint glass with a high index of refraction)
- Amici prisms (restores dispersed light to white light or vise versa)
types of samples can be measured with the Abbe-refractometer?
- liquid
- lower n than measuring prism (snell’s window can from shadow lines)
role of the Amici prism?
rainbow to white
optical dispersion?
white to rainbow after prism
Factors influencing the value of index of refraction
- Concentration (linear relationship)
- Temperature: (inverse relationship)
as temperature increases, the density of a liquid decreases, speed of light increase, lower index of refraction
Formation of Snell’s window
- lower n to higher n
- light refracts towards normal as to enters higher n
- Light comes in from many angles → we can see image of lower index medium at a broader angle
How do you determine concentration by refractometry?
Plot graph (index of refraction (y) of solution over conc of solution(x))
Trace to conc at certain n
n1 = n0+Kc To find the exact value (find c)
Definition of absorption spectrum
- Absorbance as a function of wavelength of incident light
- Different peaks (max absorbance) indicate that different molecules need a certain amount of energy to be excited
- Unique for different elements because of different electronic structure
What information can you obtain from an absorption spectrum?
Know what at wavelength is absorbance max
How do you determine concentration by absorption photometry?
Beer lambert’s law
A = log(J0/J) = Ɛcl Find c
Ɛ: molar absorptivity
