A2 Advancing Physics

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    Name: Giorgio Muscat

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    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.