Ch 10 - Colour vision and colour theory Flashcards
what is happening w white vs black objects
white - reflecting all visible light
black - absorbing all visible light
4 processes that happen when people see colour
- light source generates light
- light that is not absorbed by obj either passes through or reflects
- structures in the retina absorb light
- electrical signals are transmitted to and interpreted by the brain
what does light do to electrons in structures?
causes transitions between quantum states
opsins
proteins in the retina (back of the eye) that sense visible light
most humans have 3 diff opsins in ours: long wavelength sensitive (“red”, peak 564. - 580 nm), middle wavelength sensitive (“green” peak 534-555 mm), short wavelength sensitive (“blue”, peak 420-440nm)
each can absorb different wavelengths of light, bc multiple electron transitions possible so difference cone cells excited and we see a range in colours
chromophore, aka
aka photopigments
components of chemical structure that absorb or emit light
what vitamin are structures of many chromophores based on
vitamin A, retinal
what type of colour vision do people have?
trichromatic colour vision
how to retinal like structures interact w visible light?
many conjugated bonds and in some conjugated systems quantum states are available that correspond w energy of visible light.
variation in opsin environment of retinol allows to see red, green and blue
how do we see colours other than red blue and green
brain uses signals from two or more cones construct our perception of each colour (ex see yellow when red and green cones stimulated)
diff between colour wheel/how we perceive light and visible spectrum
visible spectrum shows different colours corresponding to different wavelengths while we see many non spectral colours which are constructed by our brains from lights of different wavelengths (ex magenta a mix of blue and red light, not rep by a specific wavelength on the electromagnetic spectrum)
additive colours
colours created by mixing light w different wavelengths
RGB Numbers
A way colours are often represented, each red blue and green goes up to 255
CMYK systems
printers etc use cyan, magenta, and yellow to create colourse, inks subtract colour from the paper. cyan absorbs red light from paper so appears cyan (blue and green), magenta absorbs green light so seen as magenta( blue and red) yellow absorbs blue so appears combo green and red (yellow)
use subtractive colour mixing
colour blindness
ppl may not have genes to produce all 3 opsins, retina could be damaged or there could be damage in areas of the brain responsible interpreting colours
X linked, so more common in men because women have backup (x linked recessive)
conjugated systems
alternating multiple and single bonds
p orbital interactions lead to delocalized electrons, shared by more than two atoms in the system. this leads to planarity and single bonds being shorter than normal and double/ triple bonds longer than normal
1,3-butadiene
important organic molecule
2 carbon carbon double bonds sep by one c-c single bond, planar
sp2 hybridized carbons so trigonal planar
what does sigma bonding help determine as opposed to pi
sigma determines overall shape of the molecule pi bonding more relevent to electronic properties and reactivit of molecule
how to determine whether a molecular orbital is bonding or antibonding
look at bonding and antibonding regions, if more bonding regions bonding, more antibonding regions then *, antibonding
how to use mo theory to determine bonding etc for a molecule
figure out hybridization scheme - what atomic orbitals are left over???
draw out what different pi orbitals could be formed based on orientation of unhybridized orbital
write out according to energy levels of different orbitals, lowest has least nodes and up based on amount of nodes
how many electrons? fill the orbitals based on regular orbital filling rules
why are all atoms in conjugated system on the same plane?
cant spin because this would break the bonds
equation for energy of particle in a box
n^2 * h2 / 8m*L^2
equation for change in energy between quantum states
(nf^2 - ni^2) * h2 / 8m*L^2
how to apply energy equation particle in a box to electrons in MOs finding wavelength of light
- determine the length of the bond system based on the radius of atoms etc
- apply the equation, transitions if MO1, n = 1, mo2 n = 2 mo3 n = 3 electron weight etc, use length of the bond system as L
- use energy calculated and the equation E = hc/wavelength to determine the wavelength (wavelength = hc/E)
equation for length of a conjugate system
L= square root of [((nfinal^2 - ninitial^2)h^2wavelength)/8mh*c]
how can you calculate length of a conjugate system based on other equations
use change in E = hc/wavelength = equation for change in energy of a conjucate system, rearrange for L
molecular vibration
atoms in bond oscillate as if attached to a spring
energy spacing between quantized vibrational states correspond to infrared part of electromagnetic spectrum (extends just past red visible region, slightly longer wavelength of about 1 mm)
infrared light
wavelength approx 1 mm extends just past red on visible spectrum
generally detected as heat
comes from molecular vibrational transitions
infrared spectroscopy
produces spectra, allows for bond types in chemical compounds to be identified
y axis displays transmission, at a maximum when absorption is minimal
x axis cm^-1, wavenumber, related to frequency and wavelength. Wave number = 1/wavelength
because measures transmission, peaks pointing down (so troughs) rep where molecule absorbing energy
bonds involving hydrogen occur near 3000 cm^-1, C-C, C-O, C-N occur near 2000 cm^-1
rotational energy
aka angular kinetic energy: kinetic energy due to the rotation of an object (or electron), part of it’s total kinetic energy
rotational states
characterized by angular momentum
can solve the schrodinger equation to find wavefunctions used to describe states of rotating molecule
energy differences between quantized rotational states corresponds to electromagnetic radiation in the microwave region
microwave waves and rotational states
energy diff between quantized rotational states corresponds to electromag radiation in microwave region
wavelength around 1 m to 1 cm length
how does a microwave work?
microwaves match energy required to transition from ground state to excited rotational states
during relaxation back to ground, heat is emit, heats up everything around
matches rotational energy transitions in water, so food w/ high water content works especially well in microwave
quantum dots
materials that glow/emit light at various wavelengths when absorb uv light
super super small so properties governed by quantum mechanics, electrons bound to super small 3D spaces
- size of this space relates to characteristics including colour
nanocrystals (particles smaller than 100 nm in one dimension) and semiconductors (materials more conductive than insulators, not as conductive as conductors)
fluorescence
occurs when lower energy light emitted after electron excited to higher energy state
synthesis of quantum dots
done using ionic forms of the elements (ex. Ch2+ and S2-)
varying reaction time and temp controls quantum dots’ size, longer rxn time bigger dots
is radiation harmful?
only ionizing radiation which has enough energy to cause electrons removed from matter/alter structure of molecules
inc. uv, x rays, gamma rays
non ionizing are ok - inc visible, radio, microwave, infrared
summary: what regions of electromagnetic spectrum promote the following
- ionization
- electron transitions
- molecular rotation
- bond vibration
- uv, x-rays, gamma rays
- visible, most of uv
- microwaves
- infrared
if theres a triple bond in a conjugated system, how many of the bonds are part of the system?
only one of the pi bonds is part of the conjugated system
as n increases how does energy spacing change?
as n increases changes in energy increase look at pg 4.24 based on equation e = n^2 - ni^2/jhjhg n on the top, so as ^^^ E increases
mo and atoms on same plane
take into account MOs and how an atom may need to twist
