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biofiz lab > Lab topics > Flashcards

Lab topics Flashcards

1
Q

Define absorbance

A

How much light is absorbed by medium after passing through

A = log J0/J

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2
Q

Define transmittance

A

How much light is left after passing through a medium

T = J/J0 100
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3
Q

How much light is transmitted by a sample with an absorbance of 1?

A

A = log J0/J
1 = log J0/J1
J0/J1 = 10

T = J/J0 * 100 = 1/10*100 = 10%

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4
Q

Which sample transmits more light: OD=1 or OD=3? By how much?

A

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

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5
Q

How does the absorption spectrum change if the sample concentration is doubled?

A

It doubles (due to the linear relationship in Beer Lambert’s law)

log (J0/J) = Ɛ (λ) c x

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6
Q

How does the absorption spectrum change if the sample concentration is halved?

A

It halves (due to linear relationship in Beer Lambert’s law)

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7
Q

What is the absorption maximum characteristic of?

A
  • The material’s molecular structure (electron excitation energies of that material).
  • We use it to measure the concentration of a material in a solution.
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8
Q

What is the function of the monochromator?

A
  • 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.
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9
Q

Define Biot-law

A
  • 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

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10
Q

Describe the linearly polarized light.

A

An electromagnetic wave, in which the electric field line vector propagates in one plane only.

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11
Q

What light source is used for polarimetry and why?

A

Monochromatic light is used because the specific angle constant is unique to a certain type of wavelength

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12
Q

the optical rotation angle and concentration

A

directly proportional

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13
Q

What is a chiral molecule? Provide an example

A
  • A molecule that has a non-superimposable mirror image with four different ligands
  • e.g., D-glucose and L-glucose
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14
Q

Factors influencing specific optical rotation

A
  • Wavelength (D= 589nm for sodium lamp)
  • Temp (20 deg)
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15
Q

How do you determine concentration by polarimetry?

A

Biot’s law: knowing the angle of rotation and the length of the tube (dm)

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16
Q

Refractive media of the eye. Image formation by the eye

A
  • Air - Cornea - aqueous humor - lens - vitreous humor
  • Light rays converge on the center of retina (fovea centralis)
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17
Q

What is the refractive power of the unaccommodated human eye?

A
  • 64 dioptres
  • Cornea contributes 43D and lens contribute 23D
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18
Q

Which refractive surface contributes the most to the refractive power of the human eye?

A

Cornea (43D) because of big difference in n of air (around 1.00) and cornea

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19
Q

Describe the circularly polarized light.

A
  • 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
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20
Q

Describe the process of focal accommodation of the human eye

A
  • Ciliary muscles contract ​
  • suspensory ligament relaxes
  • lens becomes more budged ​
  • Radius decreases ​
  • Power increases → allows you to see closer object
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21
Q

How do you calculate the accomodation power of human eye?

A

D = 1/op - 1/or

Op- Nearest d of clear vision
Or = farthest d of clear vision

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22
Q

How would you measure the position and diameter of the blind spot?

A

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

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23
Q

What is myopia and how do you correct it?

A
  • Nearsightedness, elongated eyeball, image focused behind retina
  • Can see close but not far
  • Divergent lens needed (concave)
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24
Q

What is hyperopia and how do you correct it?

A
  • Farsightedness, shortened eyeball, image
  • formed behind retina
  • Can see far but not close
  • Convergent lens needed (convex)
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25
Q

How does the refractive power of the human eye change during accommodation?

A

Refractive power (D) of human eye increases
- because radius of the lens decreases

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26
Q

What is visual acuity and how do you measure it?

A

1 min/limiting angle in arc min = visual acuity in visus

Visual acuity: how sharp eyesight is (ability to detail image)

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27
Q

How did we measure visual acuity?

A

Using Landolt’s broken ring where

Limiting angle (rad) = size of break/ distance to ring

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28
Q

Describe the reduced eye model

A
  • 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
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29
Q

Factors influencing visual acuity.

A
  1. Irregular shape of the lens and the eyeball
  2. Diffraction (causing airy disks)
  3. Anatomical density of photoreceptors (rods and cons)
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30
Q

Spatial distribution of photoreceptors on the retina

A
  • 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.
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31
Q

How do you determine the spring constant of a cantilever?

A
  • hooke’s law: F = -kx
  • plot displacement vs force
  • slope is spring constant
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32
Q

What is the visual acuity of a patient with a limiting angle of vision of 2’

A

½ * 100 = 50%

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33
Q

Parts of the scintillation counter

A

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

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34
Q

Sources of noise in the scintillation counter

A
  • external noise (ie. background radiation)
  • internal noise (ie. from electronics):
    higher V = more noise
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35
Q

How can external noise be reduced in scintillation counting?

A

lead shielding (place isotope in a lead holder with a narrow slit called a collimator serves this purpose)

36
Q

How can internal noise be reduced in scintillation counting?

A

Using the minimum required voltage of the photomultiplier

37
Q

Describe the function of the integral discriminator

A
  • Signal-selecting device that ignores pulses below a certain amplitude
  • filter out noise ​​improving the signal-to-noise ratio.
38
Q

Define the signal-to-noise ratio

A

Ratio of useful signal and useless noise
higher value = more accurate reading

Signal to noise ratio = number of signals/ number of noise pulses.

39
Q

How do you find the optimal integral discriminator setting of the scintillation counter?

A

Highest signal to noise ratio, voltage already set

40
Q

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?

A

2^8 -> 256 electrons

Exponential because every collision with the dynode produces a secondary electron (double at every collision)

41
Q

What is presbyopia and how do you correct it?

A
  • ability to see things close-up gets worse with age due to loss of lens elasticity
  • Convergent reading glasses to correct
42
Q

Define surface density

A

the amount of mass per cm^2

Unit is g/cm^2

43
Q

Define the attenuation coefficient

A

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

44
Q

Define the half-value layer thickness

A

the thickness of the material when intensity of radiation entering is reduced by one half

45
Q

Define the tenth-value layer thickness.

A

the thickness of the material when intensity of radiation entering is reduced by one tenth

46
Q

Explain the energy dependence of mass attenuation coefficient in case of lead

A

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)

47
Q

Compare the linear and mass attenuation coefficients of liquid water and steam

A

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

48
Q

What fraction of intensity is transmitted through an absorber with a thickness twice its half-value layer thickness (x=2D)

A

J/J0 = e^-μx
J/J0 = e^(-ln 2)(2) = 1/4

49
Q

What fraction of intensity is transmitted through an absorber with a thickness three times its half-value layer thickness (x=3D).

A

J/J0 = e^(-ln 2)(3) = ⅛

50
Q

Harmonic oscillation (definition, equation, graph)

A

Definition: when restoring force is directly proportional to displacement

Equation: x = A sin (ωt)
A: amp, ω: angular frequency

Graph: sinusoidal wave, displacement-time

51
Q

Damped free oscillation

A
  • amp decreases exponentially over time
  • Realistic oscillation where there is a loss of energy due to friction
52
Q

Driven oscillation, resonance

A
  • 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.
53
Q

Resonance curve

A
  • shows amp-freq function
  • highest amp = eigenfrequency

x-axis: frequency (f)
y-axis: amplitude (A)

54
Q

if the oscillatory mass is doubled

A

resonance frequency decreases by sqrt(1/2)

55
Q

if spring constant is doubled

A

resonance frequency would increase by sqrt(2)

56
Q

Define the eigenfrequency

A
  • natural frequency/resonance frequency
  • system oscillates without any external force

f = 1/2pi sqrt(k/m)

57
Q

Define the mass attenuation coefficient

A

howmuch wave is weakened by the material it is passing through

unit is cm2/g

58
Q

Gain and gain level of the amplifier

A

gain: how loud
- Power gain (Ap) = output/input power
- Voltage gain (Au) = output/input voltage

gain level: log scale of gain (dB)

59
Q

Compare voltage gain and power gain.

A

Power gain
Ap = 10 log (P out/ P in)

Voltage gain
Au = 20 log (V out / V in)

60
Q

What is the gain level if the voltage gain is 1000?

A

20log(Au) -> 20log(1000) = 60dB

61
Q

What is the gain level if the voltage gain is 1?

A

20log(Au) -> 20log(1) = 0

62
Q

What is the power gain if the gain level is 3dB?

A

3db = 10log(Ap) -> 10^(3/10) -> 2

63
Q

Frequency response curve of the amplifier.

A

describes the dependence of gain level on the frequency

x-axis: log scale of f (Hz)
y-axis: power gain level (dB)

64
Q

How do you determine the transfer band of an amplifier

A

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

65
Q

How does the bandwidth of an amplifier change with negative feedback

A

increase in transfer bandwidth (wider frequency)

66
Q

Advantages and disadvantages of using negative feedback in an amplifier.

A

advantage: stable, less distortion, wide transfer band

disadvantage: decreased gain, need more amplification

67
Q

Voltage divider circuit

A
  • a simple series resistor circuit
  • Its output voltage is a fixed fraction of its input voltage determined by two resistors
68
Q

Radius of curvature (R) and refractive power (D)

A
  • inversely proportional
  • This is because D is determined by focal length which is affected by R
    D = 1/f
69
Q

radius of curvature in the case of a lens

A

distance from the vertex to the center of curvature

70
Q

index of refraction (n) and refractive power (D)

A
  • directly proportional
  • n: indicates light bending quality of medium
  • Lensmaker equation:
    D = 1/f = (n-1)(1/r1 + 1/r2)
71
Q

refractive power

A

D = 1/f

72
Q

within the focal distance of a converging lens

A
  • on same side
  • Magnified, virtual, upright
73
Q

between the single and the double focal distance of a converging lens

A
  • beyond 2f
  • magnified, real, inverted
74
Q

outside the double focal distance of a converging lens

A
  • between F2 and 2F2
  • diminished, real, inverted
75
Q

image formed by a compound light microscope

A

Magnified, inverted, virtual

Objective: magnifies on real image plane (intermediate image)
Ocular lens: magnifies to produce virtual image

76
Q

the total magnification of a light microscope if the objective magnification is 100x and the ocular magnification is 20x?

A

100*20 = 2000x

77
Q

Describe the steps of the eyepiece scale calibration process

A

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

78
Q

prisms are present in the Abbe-refractometer?

A
  • 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)
79
Q

types of samples can be measured with the Abbe-refractometer?

A
  • liquid
  • lower n than measuring prism (snell’s window can from shadow lines)
80
Q

role of the Amici prism?

A

rainbow to white

81
Q

optical dispersion?

A

white to rainbow after prism

82
Q

Factors influencing the value of index of refraction

A
  • Concentration (linear relationship)
  • Temperature: (inverse relationship)
    as temperature increases, the density of a liquid decreases, speed of light increase, lower index of refraction
83
Q

Formation of Snell’s window

A
  • 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
84
Q

How do you determine concentration by refractometry?

A

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)

85
Q

Definition of absorption spectrum

A
  • 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
86
Q

What information can you obtain from an absorption spectrum?

A

Know what at wavelength is absorbance max

87
Q

How do you determine concentration by absorption photometry?

A

Beer lambert’s law
A = log(J0/J) = Ɛcl Find c
Ɛ: molar absorptivity

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