Andres Research Essay

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Physics, Mr. Tonkinson Research essay Andrs Bereznev

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Determining the percentage of plane

polarized light reflected from a piece of

glass at the Brewsters angle

Name: Andrs Bereznev

School: The British School Quito

Supervisor: Mr. Paul Tonkinson

Subject: Physics

Date: 14/January/2014

Main article world count: 3102 wordsAbstract word count: 209 words


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Abstract

The investigation of light has been started in the 5th

century before Christ. Such properties of light as

polarization by reflection are usually the most interesting. When light changes medium under a certain

angle called the Brewsters angle, the plane polarized light reflects while the rest is absorbed by the

medium. The reflected light seems to be completely polarized. A polarized filter is a sort of filter that

allows polarized light to pass through when its interior lines are aligned to the way the transverse light

wave goes. This investigation is about how much polarized light is reflected from a piece of glass at the

Brewsters angle. To check if all the light is polarized, a piece of equipment measures the amount of

reflected light that passes through a polarized filter placed in front. This filter is rotated to gradually cut

off the chances of plane polarized light passing through the filter, allowing us to know how much of the

total reflected light is polarized. The experiment is done and the results are analyzed to compare themto a Theoretical and experimental research. At the end some explanations are done and the amount of

polarized light reflected from a piece of glass at the Brewsters angle is worked out.


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ContentsAbstract ......................................................................................................................................................... 2

Introduction .................................................................................................................................................. 4

Theoretical research ..................................................................................................................................... 6

Conclusion ................................................................................................................................................. 7

Experimental research .................................................................................................................................. 8

Explanation ............................................................................................................................................... 8

Graph ........................................................................................................................................................ 9

Conclusion ................................................................................................................................................. 9

Body/Methods/Results ............................................................................................................................... 10

Variables spider diagram ........................................................................................................................ 10

Introduction ............................................................................................................................................ 11

Aim .......................................................................................................................................................... 11

Variables information ............................................................................................................................. 11

Prediction ................................................................................................................................................ 12

Apparatus ................................................................................................................................................ 14

Diagram ................................................................................................................................................... 15

Method ................................................................................................................................................... 15

Results ..................................................................................................................................................... 19

Analysis ................................................................................................................................................... 20

Conclusion ................................................................................................................................................... 21

Illustrations ................................................................................................................................................. 22

.................................................................................................................................................................... 23

.................................................................................................................................................................... 24

.................................................................................................................................................................... 26

.................................................................................................................................................................... 27


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.................................................................................................................................................................... 28

Bibliography ................................................................................................................................................ 28

Introduction

I choose this topic because I thought it was interesting to know how many different things arein a ray of light and one of the most interesting one for me is the Orientation of the plane of

oscillation of the electric and magnetic fields that make up light. So I decided to investigate

plane polarized light. Itsinteresting for me that when light is reflected from a boundary such as

glass under a certain angle called the Brewstersangle only plane polarized light is reflected. So

I wanted to know at what degree none plane polarized light passes throw the polarized filter.

The Brewstersangle is an example of polarization through reflection.

Figure 1Polarization by reflection

The Brewsters angle is an angle at which light enters a new medium causing only plane

polarized light to be reflected and all other light to be absorbed into the medium. At an angle of

56 from the normal the Brewstersangle theory should work for the medium of air to glass.


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Figure 2Brewsters angle polarization

I think its worth to study this to know if at the Brewstersangle only polarized light is reflected.

This experiment involves using the Brewsters angle and Malus law. Brewsters angle states

that the angle at which light reflected from a piece of glass is only polarized, this angle is

around 53 degrees from the normal. When light enters a medium like glass at the Brewsters

angle, part of the light is absorbed into the glass and if the angle is completely exact, plane

polarized light is reflected. Malus law is that at a rotation of 90 degrees the plane polarized

light passing through a polarized filter should be none. As the plane polarized light that has

been reflected from the piece of glass contains transverse waves that go up and down, they can

only pass through the filter when the filter is aligned correctly. The light will not pass through

the filter if the filter is rotated 90 from the correct position, some light will pass through the

filter at half way between 0 and 90.


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Figure 3 Polarized light passing through a rotating polarized filter

My aim in the experiment is to measure the amount of plane polarized light passing through a polarized

filter and different angles of rotation. The results will show the relationship between the amount of light

passing through the filter and the angle of rotation of the filter. In expectation, at the Brewstersangle

and at perfect experiment conditions, at an angle of 90 the light passing through the filter should be 0.

Theoretical researchIn theory the Brewstersangle for a piece of glass which has a refractive index of 1.515 is 57 from the

normal according to http://micro.magnet.fsu.edu/primer/java/polarizedlight/brewster/. So when the

light is fired at the piece of glass at that angle, the light reflected from the piece of glass should be

completely plane polarized. So when the polarized filter will be rotated 90 from the position where all

the light passes, the light should be completely undetectable. If the light entering the polarized filter is

completely plane polarized light, then when you rotate the polarized filter, every 90 no light should

pass through the filter and every half way between 0 to 90 or 90 to 180 or so on, all the light coming into

the filter should pass through.


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Figure 4theoretical results of amount of light passing through the polarized filter

Figure 5Expected results

Conclusion

My experiment should give a graph that looks like a sine function when the results plotted go from 0 to

360 degrees. The minimum amount of light passing through the filter should be 0, but as the

experiment has lots of background light, the minimum amount is unknown. About half from the lightshould be polarized. The maximum result must be when the polarized filter is rotated so that the

polarization lines are parallel to the filter lines.

Amoun

toflighpassingthorughfilter

Angle of rotation of filter


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Experimental research

Explanation

Normal light is not polarized, which means that the oscillating electric field that makes light up has

multiples planes through 360. When light enters a boundary such as glass, some light can be absorbed.

This only happens at a certain angle. To make the light reflected polarized the light should enter the

glass at the Brewsters angle which is 56.3 from the normal. At this angle only polarized light is

reflected. This light passes through the filter completely when the filter line and the lines direction are

parallel. As you turn the filter the light amount decreases until the lines of the filter are perpendicular to

the light direction, which causes the minimum possible light to pass through the filter. After the

minimum is reached, the amount of light increases until it reaches to have the filter lines vertical again.

Polarization by reflection

Figure 6 Polarization by reflection

Figure 7 Polarized filter axis explanation


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Graph

Figure 8 Theoretical results

Conclusion

My experiment should give a graph that looks like a sine function when the results plotted go from 0 to

360 degrees. The minimum amount of light passing through the filter should be 0, but as the

experiment has lots of background light, the minimum amount is unknown. About half from the light

should be polarized. The maximum result must be when the polarized filter is rotated so that the

polarization lines are parallel to the filter lines.

Amountoflighpassingthorughfilter

Angle of rotation of filter


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Body/Methods/Results

Variables spider diagram

Determining the percentage ofplane polarized light reflected

from a piece of glass at theBrewsters angle

Backgroundlight

lightintensity

rotation ofthe filter

color ofpaper belowthe piece of

glass

type ofpolarized

filter


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Introduction

I am investigating how the angle of rotation of a polarized filter affects the amount of polarized light

passing through it. To make the light incoming to the filter polarized I reflected it from a piece of glass at

the Brewstersangle. The Aim of this investigation is to discover at which angle the polarized filter has to

be rotated to let no polarized light through. The independent variable is the polarized filter which I

rotated perpendicular to the direction of the light. I rotated the filter 90 anticlockwise and then

returned and went past 0 to reach 20 clockwise from the normal. The normal was the position at which

the maximum amount possible of polarized light passed through the filter which meant that the plane

polarized light is parallel to the lines of the filter.

Aim

To determine the percentage of plane polarized light reflected from a piece of glass at the Brewsters

angle.

Variables information

The independent variable in this investigation will be angle of rotation of the polarized filter through

which light will travel, it will be measured in degrees, and a recording will be taken every 5 of rotation.

The dependent variable will be the amount of light passing through the polarized filter, it will be

measured in (Lux) using a light meter set up after the polarized filter. One of the control variables will be

the color of the paper below the glass sheet; this will be controlled by pacing 6 A4 black bond papers

below the glass in order to cover the lower surface of the glass completely. The Initial light intensity is

another control variable which will be controlled by using light source of the next specifications:

Brand: Audio tech pro light

Model: AT-CF901

Rated voltage: 120v

Frequency: 60Hz

Lamp: 4515 6V/30W

Fuse: 125V/1A

Power consumption: 30W

Another control variable is the background light, for this experiment we kept the background light as

low as possible, but this control variable does not change the results pattern, it only shifts them up and

down. A very important control variable is the angle of incidence of the light source into the sheet of

glass, and we controlled it by maintaining this angle at 53.6 from the normal. Also a control variable is

setting the tube with the filter which reflected light enters to be parallel and aligned with the angle and

direction at which the light reflects.

Diagram


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Figure 9 Diagram set up

The polarized filter should face the incoming light perpendicular and we control it by setting it like this:

Diagram

A very important control variable is the starting rotation of the polarized filter. It should be turned

carefully until the light meter shows the maximum possible value. The position at which the maximum

value is shown is the position at which the experiment should be started.

Prediction


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I think that when the filter will be rotated 90 from the normal the amount of polarized light should

decrease to its minimum. So every 180 from the normal the amount of light should go down to

minimum and back to the maximum again.

Figure 10 prediction graph


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Apparatus

Light source- Characteristics:Brand: Audio tech pro light

Model: AT-CF901

Rated voltage: 120v

Frequency: 60Hz

Lamp: 4515 6V/30W

Fuse: 125V/1A

Power consumption: 30W

Piece of glass- Measurement needed 3 clamp stands Protractor Kitchen paper towel tube Polarized filter 6 pieces of A4 black paper A big teacher protractor A light measurement device A results recording computer program Scotch


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Diagram

Method

1) Take a kitchen paper towel tube and make and stick the polarized filter at one end of the tube2) Stick it without covering any of the filter itself, only the plastic borders of the filter.3) Put enough scotch when you are sticking it to not allow any light come through holes between

the filter and the tube

4) Put the 6 A4 pieces of black bond paper on the desk, and put the glass sheet carefully over themin order for the papers to cover the lower surface of the glass totally.

5) Put a clamp stand at a side of the piece of glass at exactly half the distance between the twocorners and as close to the glass as possible until they touch

6) Put the light source on the clamp stand and point it in a way in which the light incomes the glasssheet at an angle of 53.6 degrees from the normal which has to be exactly at the center of the

glass sheet. Measure this angle using a big protractor and adjust it by moving the light source up

and down and rotating it too.


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7) Put another clamp stand, opposite to the one before also at the midpoint of the corners and asclosely as possible to the glass.

8) In this clamp stand you have to attach the prepared before tube with filter. The side with thefilter has to face the glass at 53.6 degrees too. This should be parallel to the direction of the

reflected light.

Figure 11 Polarized light entering the filter


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Figure 12 Detail of the polarized light entering the filter

9) At the other side of the tube you have to insert the light meter, fixed to record the light intensityat the middle of the filter.

10)Place another clamp stand in order to hold the protractor which will be passes through the tubein order to see the rotation degree of the filter.

11)This protractor should be fixed and must not move when the filter with the tube are rotated


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12)When the apparatus are set up like this:

Then you can place your light meter into the tube and fix it to be exactly in the middle.

13)Connect the Pasco light sensor into your computer and run the Data studio software14)Launch the recording and rotate the filter slowly until the highest value is reached.15)Stop the recording


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16)Make a mark on the tube or the filter that will represent the initial position of the filtercompared to the protractor

17)Start a recording with the mark set at 0 degrees on the protractor, which should be yourmaximum value. Record for 10 seconds and then get an average value and put a comment

saying angle 0

18)Rotate the filter with the tube 5 to the left and record for 10 seconds and make an average andcall it angle 5

19)Rotate it 5 after every recording until you get to 90, this will show as the mark made at thestart being moved one quarter of a circle to the left. Remember to put a comment on every

result average saying at what angle it was.

20)The return to placing the mark you made at 0 again, this time record the same way but turning5 to the right this time.

21)Carry recording these results until you get to -20. I called it -20 because moving to the left waspositive so dont get confused.

22)Write down the data you have collected in a table of angle of rotation, measured in degrees,going from -20 to 90 in 5 steps. The second column should be amount of light passing through

the polarized filter, measured in (Lux); in this column you should fill the results obtained.

Results

Table 1 Results

Angle of rotation of the polarized filter () /

Degrees

Amount of light passing throw the filter (L) /

Lux

-20 1711,2

-15 1727,3

-10 1624,6

-5 1543,5

0 1429

0 1719,1

5 1598,5

10 1355

15 1326,3

20 1240,2

25 1095,1

30 1031,7

35 1001,2

40 930,5

45 910,6

50 799,6

55 760,7

60 746,8


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Analysis

This graph is plotted using all the data obtained during the experiment. The trend shows that there is a

negative correlation from 20 degrees clockwise to 70 degrees anticlockwise and after there is a positive

correlation starting to go back to the starting values. This shows that at 70 the polarized light is at its

minimum. The highest value was at 20 clockwise. This means that when we rotated the polarized filter

to the position of maximum light, we did it wrongly. We should have rotated the filter 20 clockwise

before starting the experiment, and then the results would match the theory perfectly.

y = 0.0024x3- 0.148x2- 13.244x + 1543

0

200

400

600

800

1000

1200

1400

1600

1800

2000

-40 -20 0 20 40 60 80 100

Lightamountpassingthroughfilte

rlux

Degree of turn of the polarized filter

Light passing through a polarized filter rotated at

different angles

65 753,8

70

75 771,3

80 790,8

85 819

90 925,2


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ConclusionThe amount of polarized light passing through the filter varies on the angle of the rotation of the filter.

When the filter is rotated 90 from the maximum value, the amount of light passing through the filter is

at its lowest value. The light calculated from the minimum value to 0 is the background light. This comes

from the not polarized light that could have been let through because of an incorrect Brewster angle. It

also comes from the room as itsnot completely dark. The investigation shows that the polarized light

passing through a polarized filter varies in quantity depending on the angle of rotation of the filter.

When the filter polarizers are parallel to the light wave, the maximum amount of light is allowed to pass.

But as these bend, less and less light passes until the polarizers of the filter are completely perpendicular

to the plane light wave which causes the minimum amount of polarized light passes through. So at 0

there is all the polarized light passing while at 90 none polarized light passes through the filter. And at

180 again all light passes while at 270 again none polarized light can pass. To work out the percentage of

plane polarized light passing through the filter we do the highest value minus the smallest value getting

the amount of polarized light. 1727.3 lux753.8 lux = 973.5 lux of polarized light. The rest of the light is

coming from background sources such as windows and reflections from surfaces. Theoretically the

results should have been reaching zero when the polarized filter is turned 90, but as the background

light is not plane polarized it could pass through the filter at any angle. My experiment worked and the

results are correct, the reason for the lowest value not to be zero is because of background light. So the

background light does change the results by shifting them up but does no change their pattern. So the

percentage of plane polarized light passing through the filter out of the total light passing through the

filter is 973.5/1727.3 or 56.36% (rounded to 2 d.p.), therefore we can say that the percentage of

background light coming into the filter is 43.64% (rounded to 2 d.p.). So the answer to the research

question is that the percentage of plane polarized light reflected from a piece of glass at the Brewsters

angle is 56.36%.

Figure 13 Background light effect


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Figure 14 Background light effect 2

The prediction, theoretical and experimental research has completely met the results and the sketch

graph looks very similar to the plotted results. But due to the background light the results have shifted

up by 700 lux.

IllustrationsFigure 1 Polarization by reflection ................................................................................................................ 4

Figure 2 Brewsters angle polarization .......................................................................................................... 5

Figure 3 Polarized light passing through a rotating polarized filter.............................................................. 6

Figure 4 Theoretical results of amount of light passing through the polarized filter ................................... 7

Figure 5 Expected results .............................................................................................................................. 7

Figure 6 Polarization by reflection ................................................................................................................ 8

Figure 7 Polarized filter axis explanation ...................................................................................................... 8

Figure 8 Theoretical results........................................................................................................................... 9

Figure 9 Diagram set up .............................................................................................................................. 12

Figure 10 prediction graph .......................................................................................................................... 13

Figure 11 Polarized light entering the filter ................................................................................................ 16

Figure 12 Detail of the polarized light entering the filter ........................................................................... 17

Figure 13 Background light effect ............................................................................................................... 21

Figure 14 Background light effect 2 ............................................................................................................ 22

Figure 15 apparatus diagram ...................................................................................................................... 23

Figure 16 Graph sketch ............................................................................................................................... 24
http://c/Users/AB/Downloads/Andres%20research%20essay%20first%20draft%20(1).docx%23_Toc377452961http://c/Users/AB/Downloads/Andres%20research%20essay%20first%20draft%20(1).docx%23_Toc377452961


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Figure 15 Apparatus diagram


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Figure 16 Grapa sketch


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Filter perpendicular to light direction diagram

Diagram showing correct position of tube towards the incoming light


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Polarized light passing a filter at 90 and at 0


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Discussing the setup of the apparatus


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Bibliography

Alienryderflex.Alienryderflex polarizer.2004. http://alienryderflex.com/polarizer/ (accessed December

15, 2013).

davidson, webphysics. Polarization by Reflection.n.d.

http://webphysics.davidson.edu/physlet_resources/bu_semester2/c27_brewster.html

(accessed October 24, 2013).

Molecular expressions . Optical microspocy primer specialized techniques.15 June 2006.

http://micro.magnet.fsu.edu/primer/java/polarizedlight/brewster/ (accessed 11 January, 2014).

Results of the experiment


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Physics classroom. The Physics classroom .n.d.

http://www.physicsclassroom.com/class/light/u12l1e.cfm (accessed December 15, 2013).

Physics hand book. Physics handbook.n.d. http://www.physicshandbook.com/laws/maluslaw.htm

(accessed Noviembre 20, 2013).

Spie. Optipedia.n.d. http://spie.org/x32370.xml (accessed Noviembre 21, 2013).

Tonkinson, Paul, interview by Andrs Bereznev. Brewsters angle, Polarized light, Polarized filter, Malus

law(Tuesdays December, October, November 2013).

Wikipedia. Brewsters angle.n.d. http://en.wikipedia.org/wiki/Brewster's_angle (accessed October 24,

2013).

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Andres Research Essay