Midterm II

Question 1

What is Hooke’s Law?

Answer 1

F = -k * x

F is force, k is spring constant, x is displacement of spring at equilibrium

Question 2

Define the accommodation power of the eye.

Answer 2

- the difference between the largest and smallest power of the eye

• in other words: the difference between the power of the eye when focusing on the near point (shortest focal distance = highest power) and the far point (longest FD = lowest power)

Question 3

What is the ‘normal value’ of the near point of vision in adults?

Answer 3

25 cm

Question 4

How is the power of an eyeglass lens determined?

Answer 4

D_lens = D_corrected - D_uncorrected

Question 5

What is used to designate the near point and far point in the equations for determining accommodation power or lens power?

Answer 5

O_p is the near point, O_r is the far point

they correspond to the nearest and farthest focal distances of the eye

Question 6

What is normal sight called and what characteristics of the eye allow it?

Answer 6

Emmetropia

an elastic lens and normally round eyeball

Question 7

What is the name of the condition when only close objects can be focused on?

What characteristics of the eye cause it?

What kind of lens corrects it?

Answer 7

Myopia or nearsightedness

• caused by lengthening of the eyeball
• divergent or negative lenses correct myopia

Question 8

What is the name of the condition which only allows far objects to be in focus?

What characteristic of the eye causes it?

What kinds of lens corrects it?

Answer 8

**Hyperopia **or farsightedness

• a **shortened **eyeball causes it
• a convergent **(positive)** lens corrects hyperopia

Question 9

What is the value of 1 radian?

And how many degrees is 1 radian?

Answer 9

2π

360 degrees

Question 10

How many arc minutes is in one degree?

Answer 10

**60 arc minutes **per degree

Question 11

What is the normal limiting angle of view in a healthy eye?

Answer 11

1 minute

or

0.0167 degrees

or

0.00029 radians

Question 12

What is visual acuity? What else is it known as?

How is it calculated?

Answer 12

• visual acuity is the resolving power of the eye
• AKA visus
• it is calculated as the ratio of the normal limiting angle (in minutes) to the actual limiting angle, in percents

visual acuity = 1’/⍺’ x 100%

(where alpha is the actual limiting angle in minutes)

Question 13

What is the Maxwell Boltzmann distribution?

What parameters are graphed on its two axes?

How does an increase in temperature change this graph?

Answer 13

• a probability distribution of particle speeds (or energies) in an ideal gas which shows that at a certain temperature, certain numbers of particles are likely to have certain energies
• y-axis is probability or # of particles
• x-axis is particle speed or energy
• as temperature increases the peak of the curve lowers and moves rightward on the x axis, indicating more particles of high energy

Question 14

What does the area under the curve of a Maxwell-Boltzmann distribution represent?

How does temperature affect it?

Answer 14

• the area under the curve represents the total number of particles
• temperature has no effect on this area, because it does not change the # of particles in the system

Question 15

Define simple harmonic oscillation.

Answer 15

Oscillation with sinusoidal variation in which the restoring action is proportional to the deviation.

Question 16

Define damped and undamped free oscillation.

Answer 16

undamped - oscillation without energy loss (e.g. due to friction) which is therefore constant

damped - oscillation with energy loss and decrease in amplitude over time

Question 17

Define eigenfrequency.

How is it related to displacement? What determines it?

Answer 17

the frequency at which a free oscillatory system oscillates

• independent of displacement
• determined by characteristics of the system

Question 18

What is critical damping?

How is the oscillation described in this case?

Answer 18

• when energy lost per period is so large that the system reaches equilibrium without passing through the equilibrium point
• oscillation here is aperiodic

Question 19

What formula can be used to calculate minimum photon energy to create electron-positron pair?

Answer 19

E = 2mc²

Question 20

What is damped driven oscillation?

What happens to frequency and amplitude over time with DDO?

Answer 20

• oscillation with energy loss and an outside force acting on the system, replacing the energy loss
• eventually frequency of system equals frequency of outside force, amplitude becomes constant

Question 21

Define resonance in relation to oscillatory systems.

Answer 21

• driven oscillation where otuside force’s frequency is close or equal to system’s eigenfrequency

Question 22

What is on the x and y axis of a resonance curve?

Answer 22

Y axis - amplitude

X axis - frequency

Question 23

Define the photoeffect.

How does the enrgy input relate to the energy output?

Answer 23

a gamma-photon removes any electron from the bound electrons of an atom by giving its complete energy to that electron

(kinetic energy of the removed electron equals nearly the incident photon energy)

Question 24

Define Compton Scatter.

What MCQ from a past paper relates to this?

Answer 24

• gamma photons remove “Compton electron” from outer shell of atom
• Compton photon of lower energy is emitted
• energy of gamma photon split between Comtpon e- and Compton photon

MCQ: During Compton Scatter…

A: …photons scatter on outer electrons of atoms.

Question 25

Between what two states does a molecule go in phosphorescence?

Answer 25

Between **non-excited singlet state (S0)** and **triplet state (T1)**

Question 26

What occurs in pair production?

Answer 26

A gamma photon of high enough energy is absorbed **near the nucleus** and an **electron-positron pair is created**

Question 27

What occurs in scintillation?

Answer 27

an ionizing particle (beta particle) or high energy photon (X ray/gamma photon) causes a light flash in scintillator material which can be used to detect the particle/photon

Question 28

What is Planck's constant? And what letter represents it in equations?

Answer 28

6. 63 x 10^-34 J \* s - represented by h as in E = hf or h(c/λ)

Question 29

What is the formula for absorbance?

Answer 29

**A = log J₀/J** J₀ is incident intensity, J is attenuated intensity

Question 30

What is the phase relationship between the exciting and produced photons in _stimulated emission_?

Answer 30

in stimulated emission, photons of the **same phase** as the exciting photons are produced

Question 31

How is population inversion maintained in the creation of laser light?

Answer 31

by external energy input

Question 32

What is a transition that does not produce electromagnetic radiation?

Answer 32

**internal conversion**

Question 33

In the spectrum of an x-ray source what determines the position of the spectral lines?

Answer 33

the material of the anode

Question 34

What kind of crystal was used in the scintillator for the nuclear medicine experiment?

Answer 34

a Thalium-doped Sodium Iodide crystal NaI(Tl)

Question 35

Describe the basic layout of a scintillation detector. What are its three main parts and their components? And the processes that take place in each?

Answer 35

1. **Scintillator **- NaI (Tl) crystal, converts E_kin of electrons produced by in secondary processes to light scintillation via fluorescence of Tl in crystal 2. **Photomultiplier **- semitransparent photocathode converts light from scintillator to e- flow via photoelectric effect, dynode array connected to +voltage multiplies e^-s, anode collects e^-s 3. **Analyser/counter **

Question 36

Draw a scintillation detector set-up.

Answer 36

Question 37

What is secondary emission?

Answer 37

When a primary electron of high speed hits a metal surface (dynodes), several secondary electrons are emitted

Question 38

What does an integral discriminator do?

Answer 38

lets through only pulses above a certain amplitude in order to fliter out low amplitude noise

Question 39

Define luminescence.

Answer 39

Radiation that occurs due to the transition of an electron from an excited to a ground state

Question 40

What are the two categories of bonds and the bond types in each category?

Answer 40

**Primary Bonds:** 1. **Covalent** - common electron state around participating nuclei 2. **Metallic **- electromagnetism between conduction electrons and positively charged metal ions 3. **Ionic **- Coulomb forces between ions **Secondary** 1. **Van der Waals** - btwn atoms without permanent dipole moment, temporary dipole interacts w apolar molecule and converts it to an induced dipole 2. **H-bond **- H atom interbridges N, O or F 3. **hydrophobic interaction** - weak Van der Waals

Question 41

What is Kasha's Rule? What does this mean for the emission wavelength?

Answer 41

- photon emissions of appreciable yield only occur from the lowest excited state - emission wavelength is independent of excitation wavelength

Question 42

Describe the basic steps of the mechanism for atomic force microscopy.

Answer 42

1. Van der Waals forces between the atoms of a sample and a very sharp probe tip on a cantilever bend the cantilever 2. lasers projected onto the cantilever measure this bending 3. the sample is scanned by moving the sample or the tip to cover the desired surface of the sample

Question 43

What are the 3 operating modes of atomic force microscopy?

Answer 43

1. **contact** - tip touches surface of sample, cantilever deflection relates to surface topography (for soft biological samples) 2. **non-contact** - tip is oscillated without contact to surface 3. **oscillating** - cantilever is oscillated near its resonant frequency

Question 44

What are the properties of laser light?

Answer 44

1. **Low divergence** (parallel beams) 2. **Small spectral bandwidth **(monochromicity 3. **Large power** 4. **Often polarized** 5. **Possibility of extremely short pulses** 6. **Coherence**** **(waves with constante phase difference)

Question 45

What is the shape of water?

Answer 45

icosahedron

Question 46

What light phenomenon is responsible for the sky's blueness?

Answer 46

- red light is refracted more than blue, so more blue light reaches our eyes

Question 47

What does the velocity of an electron emitted from an alkali metal due to illumination depend on?

Answer 47

the wavelength of the illuminating light

Question 48

What interactions are important in determining protein structure?

Answer 48

covalent bonds

Question 49

What does the optimal integral discriminator setting on a scintillation counter yield?

Answer 49

the highest possible signal-to-noise ratio

Question 50

What does the optical resonator in a laser consist of?

Answer 50

two mirrors

Question 51

What does the Lambert-Beer law relate to? What variables does it take into account? What is its equation?

Answer 51

the Lambert-Beer Law states that the **absorption of a dilute solution is dependent on its concentration** - includes **concentration** (c), **sample thickness** (x) and **molar extinction coefficient** (E-lambda) which depends on wavelength of applied light - the equation is **log J₀/J = E-lambda \* C \* x**

Question 52

What are two other terms for absorbance?

Answer 52

**optical density** and **extinction**

Question 53

What does Abbe's principle say about image formation in a microscope?

Answer 53

an image is formed only if at least the principle and first order maxima are captured by the objective lens

Question 54

What does the maximum point of the spectrum of luminescent radiation depend on?

Answer 54

the chemical quality of the emitting material

Question 55

During what state transition does fluorescence occur?

Answer 55

transition from excited singlet state (S1) to singlet ground state (S0)

Question 56

What kind of bonds bind the atoms WITHIN a water molecule?

Answer 56

covalent

Question 57

What is a light source that is not based on luminescence?

Answer 57

tungsten filament lamp

Question 58

What is stimulated emission induced by?

Answer 58

Spontaneously emitted photons

Question 59

Draw the optical resonator of a laser system.

Answer 59

Question 60

What else can luminescence be described as and why?

Answer 60

**cold light** - because unlike thermal radiation it does not involved the emission of heat

Question 61

# Define Stokes Shift. Represent it graphically.

Answer 61

- the difference (wavelength or frequency units) between band maxima of absorption and emission spectra - in other words, the energy difference (which is lost via heat) between the absorbed and emitted photon

Question 62

What is stimulated emission?

Answer 62

the process by which an atomic electron (or an excited molecular state) interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field - a photon created in this manner has the same phase, frequency, polarization, and direction of travel as the photons of the incident wave

Question 63

Draw the reduced eye model. What is the refractice index (n) of the eye in this model? And the distance between the nodal point and the retina, where the image is formed? And the radius of curvature between the nodal point and the eye surface?

Answer 63

refractive index (n) = 1.34 distance K to retina = 17 mm r = 5.1 mm

Question 64

How can eV be determined if given wavelength?

Answer 64

first use **E = h \* (C/λ)** to determine energy in Joules then use **1.6x10^-19 Joules per eV** conversion factor to get eV

Question 65

What are the letter designations of the electron shells starting from n=1?

Answer 65

n=1 is K, n=2 is L, n=3 is M and so one

Question 66

How can power density be determine mathematically if given the wattage of a laser and the diameter of its point?

Answer 66

Calculate the area of the point using πr² then divide the wattage by the area to get w/m²

Question 67

How can the number of photons of a certain wavelength emitted by a certain power of laser over a given time be calculated?

Answer 67

1. Use **P = ΔE/t** to get total E in joules 2. Find E of one photon using **E = h\*c/λ** (with given wavelength) 3. Divide total E by E of one photon

Question 68

How can the velocity of an electron accelerated by a certain voltage be calculated?

Answer 68

* use **E_kin = 1/2mv**² rearranged to get velocity * use conversion factor of **1 eV = 1.6x10^-19 J** to get E from voltage * mass of 1 e- in kg is **9.1x10^-31 kg** * give answer in m/s

Question 69

How can wavelength of a certain matter wave be determined with only its velocity and mass?

Answer 69

Using the **de Broglie equation** ## Footnote **λ = h / m\*v**