Nature of light

Question 1

what is wavelength

Answer 1

the distance between two peaks of the wave

Question 2

what is frequency

Answer 2

the number of times the wave passes through a fixed point every second

Question 3

what is the equation relating wavelength, frequency and energy of a photon of light

Answer 3

E = hc/λ = hv
h = planks constant (J/s)
λ = wavelength (m)
c = speed of light (m/s)
v = frequency (Hz)

Question 4

high energy radiation has __ wavelengths and __ frequencies

Answer 4

short wavelengths and high frequencies

Question 5

low energy radiation has __ wavelengths and __ frequencies

Answer 5

long wavelengths and low frequencies

Question 6

what is amplitude

Answer 6

the maximum value the electric or magnetic vector can have

Question 7

what is a phase

Answer 7

starting point in time for a wave

Question 8

which electromagnetic waves have ionising power

Answer 8

x-rays and gamma rays

Question 9

what happens if light interacts with molecules via scattering

Answer 9

direction of propagation changes

Question 10

what happens if light interacts with molecules via absorption

Answer 10

energy is transferred to the molecule

Question 11

which part of the molecule absorbs energy

Answer 11

chromophore

Question 12

what happens to absorbed energy

Answer 12

converted to heat
lost through collisions with other molecules
re-emitted through fluorescence

Question 13

what does the major energy levels of a molecule correspond to

Answer 13

electronic energy levels

Question 14

what are vibrational levels

Answer 14

molecules which posses energy that is superimposed on the electrical energy levels

Question 15

what is the equation for wavelength

Answer 15

λ = hc/E1-E2

Question 16

what is an absorption spectrum

Answer 16

the plot of probability of photon absorption vs wavelength

Question 17

what is the molar extinction coefficient (ε)

Answer 17

the probability of absorption at a single wavelength

Question 18

what is the beer lambert law

Answer 18

A = εcl

Question 19

what steps are involved in measuring single wavelengths

Answer 19

• measure transmission of solvent alone
• measure transmission of sample
• subtract 2 measurements to get absorbance

Question 20

how can the environment effect chromophore properties

Answer 20

primarily determined by the structure of the molecule
polarity of solvent or neighbouring molecules
pH
relative orientation of nearby chromophores

Question 21

what is required for movement of a photon from σ to σ*

Answer 21

absorption of a photon with a wavelength outside the uv-visible range

Question 22

how does polarity effect chromophore properties

Answer 22

polar chromophores: λmax shifts in polar hydroxylic solvents

Question 23

what is hypsochromic shift

Answer 23

transitions occur at shorter wavelengths

Question 24

what is bathochromic shift

Answer 24

transitions occur at longer wavelengths

Question 25

how does pH effect chromophore properties

Answer 25

determines ionisation state of ionisable chromophores

Question 26

does increasing delocalisation increase bathochromic or hypsochromic shift

Answer 26

bathochromic shift

Question 27

how does orientation effect chromophore properties and why

Answer 27

absorbance (ε) of a nucleic acid decreases (hypochromism) when the nucleic acid is in a single stranded polynucleotide then decreases further in a double strand because base stacking and H bonding decrease the value of ε by decreasing polarity

Question 28

what is fluorescence

Answer 28

when a molecule absorbs a photon of light a lower energy photon is emitted which is a fluorophore which is more sensitive and occurs at a lower frequency than original absorbance

Question 29

does it take longer to reach the ground state during fluorescence or regular absorption

Answer 29

fluorescence

Question 30

what is stokes shift (draw graph)

Answer 30

the shift to a longer wavelength with excitation and emission

Question 31

what is the quantum yield Q (Φ)

Answer 31

probability of fluorescence (ratio of emitted photons to absorbed photons)

Question 32

what is the quantum yield Q (Φ) determined by

Answer 32

properties of the molecule itself local environment of the molecule

Question 33

what is nonradiative transfer

Answer 33

dissipation of energy via vibrations when the level of exited state and ground state overlap

Question 34

what is the most common way for energy to be dissipated

Answer 34

nonradiative transfer

Question 35

what is quenching

Answer 35

nonradiative transfer to neighbouring molecules

Question 36

what is intrinsic fluorescence

Answer 36

contained within the macromolecule due to aromatic amino acid (usually) a protein derived fluorophore

Question 37

what is extrinsic flourescence

Answer 37

added to a macromolecule (not natural) attaches to macromolecule and doesn't effect properties

Question 38

what is the λmax range for proteins

Answer 38

310-360nm

Question 39

why does tyrosine fluoresce frequently

Answer 39

because it is very weak due to quenching

Question 40

how does polarity effect the λmax of tryptophan

Answer 40

decrease in polarity will increase λmax to a shorter wavelength because when a protein is in a polar solvent because the the exited state is less stabilised than in a more polar solvent and the energy gap between the exited state and the ground state is larger

Question 41

why may a flourescence reaction not be stopped by a quenching reagent