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Transcript of A2 Advancing Physics
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8/7/2019 A2 Advancing Physics
1/15
Name: Giorgio Muscat
1
A2 Advancing Physics; Practical
Investigation Report:
Contents
The Aim of the Investigation: ................................ ................................ ................................ ............. 2
Planning the Investigation: ................................ ................................ ................................ ................ 2
The Method & Apparatus:................................ ................................ ................................ .............. 2
Experiment 1: ................................ ................................ ................................ ............................ 3
Experiment 2: ................................ ................................ ................................ ............................ 4
Experiment 3: ................................ ................................ ................................ ............................ 4Safety Risks and Hazard Prevention: ................................ ................................ .............................. 5
Laser Risks and Hazards:................................ ................................ ................................ ............ 5
Health & Safety of Milk: ................................ ................................ ................................ ............. 6
Background Information into my Investigation: ................................ ................................ ................. 6
The Results: ................................ ................................ ................................ ................................ ....... 7
Experiment 1: ................................ ................................ ................................ ................................ 7
Experiment 2: ................................ ................................ ................................ .............................. 12
Bibliography: ................................ ................................ ................................ ................................ ... 15
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The Aim of the Investigation:
My investigation aimed to examine and observe what happens to the intensity of a laser as the
concentration of a constant sol is varied at a linear rate in a fixed volume of distilled water. I also aim
to develop this investigation to also examine how the wavelength of the light source affects the
intensity of light as it passes through a constant concentration of the same sol, the variable in this
development being the wavelength of the source. I also finally aim to investigate how the pathlength of a constant concentration sol affects the intensity of the laser as it passes through the
medium.
I will record the data with a large emphasis on reducing uncertainty, and eliminating systematic
errors which may arise. I will use my physics knowledge and further research to analyse the results I
gather and draw possible relationships between my variables. I will also explain the possible
reasoning for the results I gathered as well as explaining any uncertainty values which may be
illustrated in graphs.
Planning th
e Investigation:The initial stage of my investigation was to plan what I was going to do, this is so I can utilize and
spend my time efficiently with a good understand of what I was setting out to achieve. In this
section of this report, I will show how I planned the investigation .Highlighting the hazards involved
with my practical and the safety precautions I planned to take to reduce the risks. I will also show
how I intent to achieve my aim. This will involve displaying a clear method of my investigation,
including a description of the apparatus and materials I had used and diagrams ofhow I intend to
setup my practical.
The Method& Apparatus:
In science its very important to give a detailed method so others are able to reproduce the
investigation and verify my findings under the exact same conditions. To do this, my method of myinvestigation must be described in detail to allow for this to happen.
There are three practical stages of my investigation that I plan to do. The first practical I will do, will
try to answer the first aim of my investigation. However to do this I am going to need to set up a
controlled experiment and record the data for it, for all my investigations I will carry them out in a
dark room to avoid uncertainty due to other light sources. The apparatus I will use for the first
practical include a 532nmlaser; this wavelength will produce a green light. The use of a green laser is
because the human eye is most sensitive at this wavelength in the electromagnet spectrum, this will
possibly allow for observation of the investigation to be easier to spot during the practical. In the
first investigation I will also need a sensor, this sensor will record the intensity of the laser. To
contain the liquid sol solution I will use an acrylic transparent material with a constant volume. To
control this experiment and reduce uncertainty I will use clamps to hold the laser and sensor at a
fixed distance apart, I will also keep the angle of the laser and sensor constant. The reason for this is
that a slight change in the angle of the sensor or laser could cause a change the amount of light that
is initially reflected, it could also cause the amount of light that reaches the sensor to decrease if
they are no longer inline.
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Experiment 1:
To record my first set of data I will set up the practical which is illustrated in figure 1 below. I will
have the sensor and clamp held at a fixed distance and then I will align the angle of the beam to try
and allow for as much of the beam as possible to reach the sensor panel on the sensor device. Once
this has been achieved, the sensor and source are clamped and held in the position. I will then fill
the acrylic container to a recorded high with distilled water; this is water with no impurities however
the same distilled water must be used throughout the investigation to ensure that there are no
systematic errors. I will place the container of the liquid in position and ensure the beam passes
through the liquid. I will then mark the height of the distilled water in the container, the side that
the light is entering and mark the exact location of the container in front of the laser; this is done to
decrease uncertainty for when I move the container away. This is because if I place the container at
a different angle, the initial light that hits the acrylic container could be reflected even greater, this
would make results invalid if the amount of reflection was to change as I was collecting data. The
beam of light must also enter the same face of the acrylic container each time, this is because light
may be absorbed or reflected more on a different face. This could be due to scratches in the acrylic
or possibly a greater width of plastic that the laser would need to shine through. The height of the
distilled water is marked to ensure when refilling the container the same volume of water is used.
For adding milk to my distilled water to create my sol I will be adding it as an empirical unit, this will
be done by the amount of drops. However the amount of milk added each drop must remain
constant; to do this I will use a pipette and mark a spot on the pipet that will represent each drop. I
will then fill the pipet each time to that level indicator and add to the distilled water. This method of
adding the milk to the solution would bring an uncertainty. To calculate this uncertainty, I plan to
measure the weight of each drop and plot a graph of mass against number of drops. I will then draw
a line of best fit as well as 1 other line, since the gradient of this line is the mass per drop I should
find a linear graph. The other line I will draw will be to calculate the uncertainty; this will be done by
drawing a straight line to the point most off my line of best fit. I will then take a gradient of the line;
the change in the gradient will be the highest possible uncertainty that could arise when adding the
drops to the solution.
Once the apparatus has been set up like that described above, I will begin to record data. I will
initially record the Initial light intensity; this is the light passing through the container and liquid but
without the milk suspended within it. I will then add a single drop marked on my pipet, I will then stir
the solution to ensure that the particles of the milk are evenly distributed within the liquid. Once this
is done I will then record the change in the intensity and note any observations I see as I do so. I will
then continue to add drops until the intensity change for each drop is very small, this is predicting
that I will find an exponential relationship. If not I will add drops till I feel Ihave enough to plot a
sufficient graph.
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Figure 1: Diagram of Apparatus Setup for Varying Concentration:
Experiment 2:
To develop the investigation I will repeat the exact experiment shown in figure 1, however this till Iwill change the wavelength. By keeping everything else the same as in the first experiment, I should
be able to use the results from this development to draw relationships between wavelength and
intensity. In this experiment I will use a wavelength laser of632.8 nm; this will be a red laser. This
laser has large health and safety precautions that are described in the risk assessment section below.
Experiment 3:
The final development to my investigation is to also find out how the path length of a sol of constant
concentration affects the intensity of the light. This experiment will be done differently to that
described for my first investigation. To do this I will create a stock solution of sol with a set
concentration. I will then change the layout of apparatus; the setup is shown in Figure 2 below. The
major change made is that I will now place the sensor and laser in a vertical path instead of ahorizontal. I will then place the acrylic container again the path of the beam. I will keep the path
distance constant again and position of the container constant.
Once the apparatus is set up I will then begin to take results again, this time the variable will be the
path length of the solution. I will vary the path length by adding the solution of milk / distilled water
to increase the height of the liquid. The path length will increase as I had the solution, but the
concentration will not increase. Before adding any solution I will record the initial intensity again
however in this experiment it will be done with a path length of 0 (No solution to pass through). I
will then increase the path length; this again will be done using empirical units. I will add the same
amount of the stock solution each time it is added; by adding the same amount of liquid each time I
will increase the path length by the same amount each time. I will record data for each increase ofthe path length, as well as note down any observations that I notice during the experiment.
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Figure 2: Diagram of Apparatus Setup for Varying Path Length:
Safety Risks and Hazard Prevention:Like a lot of scientific investigations, there are possible health and safety implications that may arise.
I will address the hazards that my experiment may create and I will also describe how I can decrease
this hazard.
Laser Risks and Hazards:
The main hazard that is involved with my investigation is due to the use of a laser. This hazard can be
completely avoided by the use of a typical LED. However for my investigation I wanted a focused
beam of light. The use of a LED will cause the light to decrease in intensity severely due to the
inverse square law, the is due to the light being emitted in a cone shape towards the sensor.
Although the intensity due to the inverse square law will be constant as the path length is constant I
wanted to have a direct beam though my solution, this is so I can make observations as to whathappens to the light as it passes the sol. This is much easier to do with a laser then a typical LED as
the light is spread out a lot more with an LED.
Light emitted from a laser can be very hazards; this is because if the light directly enters someones
eye it can easily cause damage, even low powered lasers like that used in my experiment can cause
serious damage to an eye, this is due to beam being focused on a very small point of the retina.[1]
Therefor it was important for me to address this serious issue, the first step to decreasing the risk of
this hazard was to isolate the laser from other pupils, this was done by carrying out my investigation
in an almost isolated environment in a dark room. Although 1 other student was inside the dark
room also, however this student was fully aware of the risk of the laser. To increase safety further I
ensured that the laser was clamped, this is to ensure that the laser doesnt move during myinvestigation. This is very important for collecting accurate data, but it also reduces the chance that
the laser could be knocked out of its normal path and possibly into someones eye. The final step I
did to decrease the risk of the laser was to have the laser facing towards a wall, or towards the floor
(Experiment 3). Byhaving the beam facing the wall, I was able to avoid anyone accidentally walking
into the beam. This is also shown in figure 1 & 2.Experiment two required the use of a different
wavelength laser, the only laser available to me at this stage and with limited time was a very
powerful laser. For health and safety reasons adult supervision was required when using this laser.
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Health& Safety of Milk:
Another hazard that can also be removed is the health and safety hazard caused by using milk to act
as a sol. This milk can go bad quickly if left out over a space of a few weeks and as a result can create
some hygiene issues. To decrease this, risk the milk used for the investigation will be sealed and
stored in a refrigerated environment after each use. Milk used during testing will be disposed of and
the container cleaned after every session of use, and spilt milk will be cleaned up. Each time I refill
the container with distilled water I will ensure that the water level is to the marked indicator each
time to keep a constant volume of water. To avoid the temperature of the milk becoming a possible
systematic error, I will allow the same amount of time for the milk to warm up each time I use the
refrigerated milk. This will reduce any chance of a possibly systematic error occurring due to
different temperature values of the sol.
Background Information into my Investigation:
By making a solution of milk and water the particles within the milk will cause light to be scattered.
For me to get a better understand of what may occur in my experiment I did some research into the
scattering of light.
From my research I found out that the radiation from my laser will encounter Rayleigh scattering:
Rayleigh Scattering is the elastic scattering of light or other electromagnetic radiation by particles
much smaller than the wavelength of the light, which may be individual atoms or molecules.[2]
I found out that Lord Rayleigh in 1871 derived the Rayleigh scattering law. This law applies to the
scattering of light of which the particle size is less than the wavelength of the light hitting it. The law
shows that the percentage of scattered light is inversely proportional to the 4th
power of the
wavelength.[3]
What this shows me is that if the wavelength of a light source is for example doubled, the % of
scattered light will decrease by a factor of 16. This is also the reason why we find the sky being blue.
Blue light has the smallest wavelength, there for from the law above the % of scattered will be at its
maximum at the blue light part of the electromagnetic spectrum. The light shines white light through
space, it enters the upper atmosphere and white light is a combination of all the wavelengths of the
visible light spectrum. The white light will hit particles in the air, particles like dust are larger than
the wavelength of visible light, and all the light gets reflected since all the light is reflected the light
remains white. However when the light hits for example a gas molecule, of which is smaller than the
wavelength some of the light gets absorbed. The blue light gets absorbed more than the other light
in the visible light spectrum. After the light has been absorbed, the molecule then radiates the blue
in different directions.[6]
If the particles due to the sol are smaller than the wavelength of my laser, I
should be able to see Rayleigh scattering.
From my research I also found that light can be polarised by scattering. Polarisation is where the
waves of a light source oscillate perpendicular to the motion of travel, whereas un-polarised waves
oscillate in many directions.[4]
This ability for the light to get scattered may allow me to make an
observation of the light as it is scattered in the liquid.
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The Resul
s:
Experi
ent 1:
The first set of res
ts that I collecte
was with a varying concentration of milk in a distilled water
solution. The results taken for thise
eriment were taken with care and with a high priority of
accuracy. Like mentioned in the planning section, I set up the practical e
periment to try and insure
very little uncertainty and no systematicerrors.
Th
imag
belo
sho
s evidence o
this experiment beingcarriedout:
One assumption made when recording data for this investigation was by adding milk to the distilled
water, thevolume remained constant. This assumption was made as I feel that from my research,
thechange in volume would beso small that theeffect on the intensity would be negligible, this is
mainly because the amount ofchange it would make would beso small that thesensor used would
not have been sensitiveenough to record it.
Milk Ready to be added to
the distilled water:
Laser fixed in a set position with
sensor aligned directly in front of it:
Sensor fixed in a set position with
Laser aligned directly in front.
Distilled water at a marked
height, with marked face of
the acrylic plastic facing the
laser:
Path the beam travels:
To the left of the
sensor was a
wall, this is not
shown in the
image.
Clamp holding
Laser and
Sensor in fixed
position:
Plasticstirrer to ensure
the milk solution isevenly
distributed
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-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8 10 12 14 16 18
Mass (
)
Nu
b !
"
f D! "
ps:
G aph Sh in Unc tainty F Pip t D ps:
Calculating The Uncertainty of the Drops added by the Pipette:
The first set of data I collected for this experiment was data to calculate how much of an uncertainty
is given each time I add concentration of milk to the distilled water.
These results were gathered by marking on the pipette the amount of milk I want 1 unit of
concentration to represent. I then put 1 unit on a sensitive scale;the least count of this scale was
0.01g there for the uncertainty of the scale is +- 0.005g.
I will calculate the mean mass of each drop from the gradient of my line of best fit of the graph of
data below:
Number of# ro$ s: Mass (%
) +- 0 & 005
0 0.00
1 0.08
2 0.13
3 0.20
4 0.26
5 0.32
6 0.40
7 0.47
8 0.57
9 0.65
10 0.74
11 0.80
12 0.86
13 0.92
14 1.00
15 1.08
16 1.16
17 1.24
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Gradient of trend line will give me the average mass of concentration that is added each time to the
solution. The gradient of the line is calculated by doing the change in mass divided by the change in
number of drops. From using point 3 where the line cuts straight through the data point, to point 17
where it reaches the lower bound uncertainty value from the sensor, I am able to calculate the mean
mass of the pipet drop.
From calculating the gradient of the line that passes through the labelled point I am able to calculate
the largest possible uncertainty that may arise from using the pipette. This was done by using the
upper bound of the largest point of the line of best fit and the same equation as above.
Uncertainty
There for the amount of milk solution added to the container each time = 0.074 +-0.001. The
percentage of uncertainty =
[5]
The mean value of my mass added per drop = 0.074, the uncertainty to the amount added each time
is plus minus 0.001. This gives a percentage of uncertainty of:
Although I wanted to keep the amount of concentration constant each time I added milk to mydistilled water to create my sol, there is a small amount of uncertainty in the actual amount added.
However, this uncertainty is very small.
After I had recorded the results for the mass per drop graph, I then started to record the results of
my experiment were I varied the concentration of the milk in a fixed volume of distilled water.
(Distilled water was used as it is pure, and has no other foreign substances are present in the
water.Distilled water can be achieved by changing the state of water by adding heat to it, by doing
this we will increase the probability that the water particles will undergo there process of
evaporation. Using the evaporated water, we can capture it and allow it to return back to its liquid
state. By doing this only the water is kept.
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0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 5 10 15 20 25
Intensity of li'
ht (0-10k)
(Lux)
Numberof Pi(ette
)ro
(s in
)istilled Water
Graph Sho in Intensity ofareen laseras
concentration ofa constant volume is increased.
Table Results for Experiment 1:
Number of0 rops: Intensity (Lux)
0-10k:
Uncertainty:
0-10k:
0 4.63 0.005
1 3.82 0.055
2 3.21 0.055
3 2.69 0.0554 2.27 0.055
5 1.78 0.055
6 1.34 0.055
7 1.16 0.055
8 0.93 0.055
9 0.80 0.055
10 0.68 0.055
11 0.48 0.055
12 0.38 0.055
13 0.32 0.055
14 0.26 0.055
15 0.14 + 0.03 0.005
16 0.17 + 0.03 0.005
17 0.15 + 0.03 0.005
18 0.13 + 0.03 0.005
19 0.12 + 0.03 0.005
20 0.08 + 0.03 0.005
21 0.06 + 0.03 0.005
22 0.05 + 0.03 0.005
23 0.04 + 0.03 0.005
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The uncertainty of these results is firstly due to again the least count of intensity sensor. The sensor
was accurate to 0.01 on the 0-10k scale. This initial means that the value could actually be 0.005.
However, there was further uncertainty in my results. The value the sensor was giving me was
fluctuating, at first I thought it was because I had stirred the sol. To check that this wasnt the cause,
I waited for a longer period of time before I took the result the observation I made was that it was
still fluctuating. The initial light intensity before I allowed the light to be scattered was notfluctuating, this allowed me to conclude that there was an uncertainty in the true value of the
intensity each time a drop is added. To solve this, I took an average of each data point I collected.
What I found while doing this was that the fluctuation was about0.05 (0-10k). In the 0-1k scale the
result was no longer fluctuating, so there was no uncertainty due to a fluctuation in the results that
had to move down to the 0-1k band. This brings a total uncertainty of, this is because the
value was fluctuating by about 0.05 and the least count uncertainty from the sensor brings a total
uncertainty to 0.055.
I found a systematic error while taking these results. This is systematic error was because the
intensity had dropped by so much after the 15th drop, I had to put my sensor on a different
sensitivity setting. By doing this I noticed that the intensity had increased even though noconcentration was added, to double check it was the sensor I put it back to the 0-10k scale and
found it was normal again. This allowed me to conclude that there was a systematic error involved
when changing the sensitivity of the sensor. To calculate how much it may have changed my results I
ran an isolated test, this involved setting a low intensity and knowing the intensity it read on the 0-
10k scale. I then dropped the sensitivity to the 0-1k scale keeping everything constant. What I found
was that the intensity had increased by 0.03 (0-10k). This allowed me to conclude that for every
result taken on the new scale, I had to add 0.03 to each to remove the systematic error.
While doing the experiment I also found a few problems. The first problem I found when I was
measuring the initial intensity of light was that the laser had a power up time, what I mean by this
is that when I turned the laser on, there was a delay before it reaches its peak intensity. I found thatthe delay was about 10 seconds. To insure my results were as accurate as possible, I waited 10
seconds before recording any data after turning the laser on.
Once I had finished taking the results of experiment 1, I repeated the initial intensity of light result.
What I found was that the intensity had decreased from 4.63 (0-10k) to about 4.58 (0-10k), although
this isnt a significant change in intensity it shows that there may be some uncertainty in my results.
The reason I believe there was a decrease in the initial intensity is due to the fact that the laser uses
batteries, these batteries could have caused the intensity of the laser to decrease as there charge
decreased. To decrease the uncertainty this caused, I would change the batteries of the laser after
each session of use. To decrease the uncertainty further and also increase safety I decided that I
would only turn on the laser for enough time for it to power up, take an average of the intensityand record the result.
I am able to calculate the percentage uncertainty for my results, in science we aim for less than 5%
uncertainty. However my results are not very certain, from using the equation:
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I was able to calculate the largest uncertainty value for my results. I found that the largest
percentage of uncertainty in my results was 21%; in science this amount of uncertainty is too much
to draw an accurate conclusion from my data. However, without time restraints I could decrease the
uncertainty further. The main cause of my uncertainty was the fluctuating value when on the 0-10k
scale; this could be because of the light source I used to read the sensor. It could also be because the
sensor was not very good, with further time invested it could have been possible to repeat theexperiment however use a much more accurate intensity sensor. With a better method of collecting
the results, this is because for this experiment due to the dark environment of a dark room I had to
use an external light source to read the recorded value.
Observation found in Experiment 1:
The key observation I made during this experiment was that as I increased the concentration of the
sol, I found that the liquid solution would glow brighter. Without any milk mixed with the distilled
water there was just a beam of light passing through the liquid, once the milk was added the liquid
began to glow. Possible reasoning for this observation ishighlighted in the analysis section.
Experiment 2:
The second experiment that I did was very similar to experiment 1.The only change I made was to
remove the green laser and substitute it for the red laser. Due to health and safety reasons, this
laser required permission to be used. After assessing the safety of using this laser, it was concluded
that it was safe to use underadult supervision, this involved having my physics tutor use the laser,
while I collected the data. The collection of the data was conducted in the exact same conditions as
experiment 2.
Table of Results for Experiment 2:
Number of1 rops: Intensity (Lux) 0-10k: Uncertainty: 0-10k:0 5.90 0.005
1 5.35 0.055
2 4.65 0.0553 4.05 0.0554 3.45 0.0555 3.27 0.0556 3.00 0.0557 2.70 0.0558 2.32 0.0559 2.10 0.055
10 1.80 0.05511 1.60 0.05512 1.45 0.05513 1.32 0.05514 1.17 + 0.03 0.00515 1.02+ 0.03 0.005
16 0.84+ 0.03 0.00517 0.75+ 0.03 0.00518 0.72+ 0.03 0.00519 0.65+ 0.03 0.00520 0.55+ 0.03 0.00521 0.49+ 0.03 0.00522 0.40+ 0.03 0.00525 0.27+ 0.03 0.00526 0.18+ 0.03 0.005
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0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Intensity of Li2
ht
(0-10k) Lux:
Number of Pipette3 rops in 3 istilled Water:
Graph Sho in Intesnity ofa Red Laseras concentration
ofa constant volume is increased.
This experiment showed the exact same problem with the sensor, to try improve my results I moved
down to the 0-1k band a lot more earlier however it still showed evidence of an increment due to
the systematic error. To remove this error, I again added 0.03 to all results recorded in the 0-1k
scale.
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=
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Bibliography:
[1] http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_Laser_Safety.shtml
This sour4 e was usedto resear4 h some risks involvedwith using a laser.
[2] http://en.wikipedia.org/wiki/Rayleigh_scattering
This sour4 e was usedfor a definition of Rayleigh s 4 attering, andfurther reading to better understand
the s4
attering of light.
[3] http://science.jrank.org/pages/5752/Rayleig h-Scattering.html
This sour4 e was usedto define the Rayleigh Law.
[4] http://www.physicsclassroom.com/class/light/u12l1e.cfm#scat
Source used to better understand polarisation of a wave
[5] Alan Stewart, Physics Tutor QMC.
Sour4e providedequation for the per
4entage un
4ertainty, advi
4e was also given.
[6] http://www.sciencemadesimple.com/sky_blue.html
Sour4e allowedgreater understanding of Rayleighs Law andhow it
4auses the sky to be blue.