GEK2507: Physical Questions from everyday Life

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GEK2507 Physical Questions from everyday Life

• Tutorial 3 Photosynthesis, Blackbody Radiation,
  Vision and Biopolymers.

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Question1 Light Harvesting Process How many
photons does it take to split one water molecule
during the photosynthesis process? In which part
of the chloroplast does the Calvin cycle occur?
Only two photons are necessary. One for each
electron harvested (water has two hydrogen atoms
whose nuclei, the H ions, fulfill important
roles. The split off oxygen combines with another
oxygen atom to form an O2 molecule).
Hence, the two photons indeed do have enough
energy to break the bond. Of course, the energy
is not directly transferred to the bond.
The Calvin cycle occurs in the stroma of the
chloroplast.
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Question2 Atomic Energy Level From Quantum
Mechanics, we know that energy levels of atom are
discrete. (a) If an atom would have three
different excited levels, how many different
wavelengths can the absorbed and emitted photons
have?
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(iii) What will happen if a photon with an energy
of 8eV strikes a Searsium atom in its ground
level? Why?
There is no energy level 8eV higher than ground
state. Therefore the electron will not absorb the
8eV photon.
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Molecules do not stand exactly still. Due to
temperature alone, they will be vibrate and thus
move towards or away from the light source. There
may also be rotational or other energies that
come into play.
Consequently, although the energy to transition
from ground state to an excited state is exactly
given, but amount of energy that needs to be
added to effect that transition is not.
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Question4 Leaves Color and Absorption (i) What
color would leaves have if they could also absorb
green light?
However, if the absorption peaks are fairly
sharp, then the colors between green and red (the
color near yellow) and the colors between green
and blue (near turquoise) would shine through.
(ii) What color would leaves have if they could
only absorb green light?
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(ii)What is the peak intensity wavelength? Is
this wavelength visible to humans?
(iii) What is the radius of Sirius B? Express
your answer in km and as a fraction of our suns
radius.
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(iv)Which star radiates more total energy per
second, the hot Sirius B or the (relatively) cool
sun with a surface temperature of 5800K? To find
out, calculate the ratio of the total power
radiated by our sun to the power radiated by
Sirius B.
Even though the absolute surface temperature of
Sirius B is about 4 times that of our sun, Sirius
B radiates only 1/39 times that of our sun. The
reason is that Sirius B is much smaller than our
sun.
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(ii) Is the radiation from the source visible? If
so, what is its color?
No, the radiation from the source is not in
between the range of visible light (400nm-700nm).
400nm is violet/blue and 700nm is the red light.
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Additional notes Interference Youngs
Double-Slit Experiment
The interference occurs because each point on the
screen is not the same distance from both slits.
Depending on the path length difference, the wave
can interfere constructively (bright spot) or
destructively (dark spot).
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Slit separation d Distance on screen x Angle
of incidence ?
x
m? ? bright (m ½)? ? dark m 0, 1, 2,
L
sin? tan? ? x/L, small ?
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Interference Youngs Double-Slit Experiment
We can use geometry to find the conditions for
constructive and destructive interference
Approx. we used sin? tan? ? x/L, small ?
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The angular separation is given by sin(?)m?/d,
which is proportional to the wavelength of the
light. Figure (b) have smaller ?, thus ?2lt505nm.
From figure, 4th order minima (m14) of green
light coincides with the 5th order maxima (m25)
of 2nd color.
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In general, the time taken is longer than this
since the light path is not parallel to the
optical fiber, but goes through many total
internal reflections.
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Question9 Reflection of Light
A small vertical mirror hangs on the wall, 1.40m
above the floor. Sunlight strikes the mirror, and
the reflected beam forms a spot on the floor
2.50m from the wall. Later in the day, you notice
that the spot has moved to a point 3.75m from the
wall. (a) Were your two observations made in the
morning or in the afternoon? Explain.
a) For the 2nd observation, since the distance
from the wall is longer, we see that the
incident/ reflection angle must be smaller (in
green). Thus, the sun is at lower latitude (local
frame) for 2nd case (setting sun). The
observation is made in the afternoon.
(b) What was the change in the Suns angle of
elevation between your two observations?
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Question10 Refraction of Light and Snells Law
The observer, as shown, is positioned so that the
far edge of the bottom of the empty glass is just
visible. When the glass is filled to the top with
water, the center of the bottom of the glass is
just visible to the observer. Find the height, H,
of the glass, given that its width is W 6.2cm.
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Although both microwave and visible light consist
of photons and are part of the electromagnetic
wave, their wavelengths are rather different. For
instance, green light is around 520nm while the
wavelength of a typical microwave is 12.24cm
(frequency of 2.45Ghz).
The door of a microwave oven has a glass panel
with a conduction perforated sheet whose holes
are a few mm in diameter. Compared to the
wavelength of the visible photons, the holes are
huge and hence visible light photons can easily
pass through. However, this hole size is
considered tiny for the microwave photons and
hence they cannot pass through.
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Both human beings and herbivores can break a
linkages since the bond is not that strong.
ß-linkages, however, are too strong for the
mechanisms available to mammals to break. This is
even so in the case of herbivores like cows and
horses. Herbivore mammals solve this problem with
the help of symbiotic bacteria that live in their
digestive systems. The bacteria produce enzymes
which break down the cellulose into simple sugars
that are then used by the mammal as food.
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The differences between amino acids lie in their
side chains. Since there are 20 different types
of amino acids in human beings, there are 20
different types of side chains.
The type of bond that holds pairs of nucleotides
such as the A-T, G-C pairs is the hydrogen bond
(relatively weak bond and easy to zip and unzip
the DNA during the DNA replication).
(iv) Name two key differences between DNA and RNA.
The base thymine(T) in DNA is replaced by the
base uracil(U) in RNA. RNA can have strong
enzymatic properties while DNA does not. DNA
generally only occurs double stranded while RNA
can be both single and double stranded and
furthermore fold in a myriad of ways. The sugar
in DNA is deoxyribose while the sugar in RNA is
ribose.
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In DNA, we have base pairing as A-T and GC. So,
percentage of Guanine is given by (1-2x0.23)/2
0.27 or 27.
No. For the functionality, it should be a given
sequence rather than a random sequence.