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1. Polarising filters
1. Polarising filters
Light in which the electric field vector oscillates in any one fixed plane is said to be plane polarised or linearly polarised. The transmission axis of a polarizer is the axis such that light with its electric field oriented parallel to this axis will be transmitted.
Light is a transverse electromagnetic wave, but natural light is generally unpolarised - all planes of propagation of the electric field vector (and therefore the magnetic field vector perpendicular to it) are possible. A polarising filter can be used to polarise light as shown below.
Try the following:
Take two polarizing filters and rotate them in relation to each other while looking through them as shown below.
If you take the brightest position as 0º, sketch a graph of the transmitted intensity as you rotate ONE of the filters 180º.
2. Hold one of the polarizing filters in front of the screen of your laptop or mobile phone and rotate it.
1. Determine the plane of polarization of the screen. (Vertical is 90 deg)
2. With your knowledge of polarized sunglasses, propose a rationale for this plane of polarization.
3. Place a transparent plastic object e.g. a clear plastic ruler, in between two polarising filters and observe the coloured patterns that form.
What could this be used for?
2. Experiment - The Law of Malus
2. Experiment - The Law of Malus
Introduction
In the 1800’s the Frenchman Étienne-Louis Malus proposed I = I0 cos2θ to predict the light transmission through two polarising filters - the polariser and analyser.
2021 Students: You will be using the data below to explore Malus Law.
2021 Students: You will be using the data below to explore Malus Law.
I0 is the intensity when the angle between the polarizer axes is zero. In this experiment you will measure the transmission of light through two polarizing filters I as a function of the angle between their axes θ and compare it to Malus’s law.
Apparatus
laptop, LabQuest2,optics kit, light sensor, light source and power supply, polarising filters (2) with rotating axes
Instructions (Confirmation Inquiry)
Place the light source, polarising filters and light sensor so light passes through the filters and then into the sensor. The filter closest to the light source is called the Polariser, the filter closest to the sensor is called the Analyser, as shown below.
You will rotate the Analyser only to change the plane of transmission; the Polariser, the light source, and the sensor must not move.
Turn on the light source.
Connect the light sensor to channel 1 of the LabQuest2. If your sensor has a range switch, set it to the 600 lux range.
Click on the Open icon and open the file “28A Polarization of Light” in the Physics with Computers folder. Illumination (intensity) of light is plotted against analyser Angle.
Rotate the analyser until the intensity reading is a maximum. If the reading is larger than 600 lux, then change the scale of the light sensor to the 6000 lux range. This is your zero angle. The axes of transmission of the two filters should be parallel.
Set the filters so their axes are at right angles. Very little light should get through the pair of filters. Define the light level as zero for this angle by clicking the Set Zero Point button . The intensity reading should now be near zero.
Return the analyser to the parallel position. Click the Start Collection button to begin data collection. Click the Keep Current Value button to take the first point and enter 0 for the angle.
Repeat this process for a range of angles until you have rotated the analyzer through half a revolution, or 180º. Click the Stop Collection button to end data collection.
Analysis
1. Describe your graph of light Intensity against Angle.
How does this compare with your predicted graph from above?
2. Click on the Curve Fit button and choose Cosine squared.
3. Click on the Data tab and choose New Calculated Column. Enter an expression for Cosine squared (cos2θ) and plot a graph of Intensity against Cosine squared of the angle. Does the graph show a proportional relationship? Are your data consistent with Malus’s law?
Copy the above data into a spreadsheet to further analyze the data.
Use =radians(Angle Cell) to convert to radians
Using the value from 90deg, 'zero' your data.
Using the value from 0 deg, create a model of the data using =0.126*cos(radians)^2
3. Polaroid sunglasses
3. Polaroid sunglasses
The reason for using polarising filters in sunglasses can be shown by comparing the light coming directly from a source with the same light when it has been reflected from a surface like a pane of glass as shown below.
Hold a polarising filter in front of one eye and look at the direct light and the reflected light. Now rotate the filter by 90° and look again at the direct light and the reflected light.
What do you notice?
What must be happening when light is reflected from a surface like glass?
Why might this be particularly useful for skiers or sailors?
4. Circular Polarisers in Photography
4. Circular Polarisers in Photography
Without a polarizer.
With a polarizer.
How to use and buy Circular Polarizers in Photography - Using the camera in the lab, observe the effects of using a circular polariser on the color and depth of the photo.
What questions does this pose in your mind? (open inquiry)
Fan of 3D movies? This is the video for you...
5. Polarizing Paradox
5. Polarizing Paradox
Observe a light through a single linearizing polarizer. According to the theory, compare the intensity of the light before and after passing through the polarizer. I=Io/2
Take a second linear polarizer and hold it perpendicular to the original. The two linear polarizers should be at 90deg to one another. Using Malus' Law, confirm your observations.
Using a 3rd linear polarizer, place it at an angle of approximately 45deg between the other two polarizers (still at 90deg). Using Malus' Law, confirm your observations.
Malus' Law
Malus' Law
Long, melodic, and very informative.
Important Considerations:
Light intensity passing through 1 polarizer is reduced by 1/2.
Focus A LOT on proportional/ ratio problems.
Sketch graph of Intensity vs. Angle
General Resources:
General Resources:
Kognity Textbook Chap 4 - Use you ACS Login
IB Physics Site: Topic 4 - Comprehensive notes
IB Physics Site: Topic 4 - More notes
Topic 4 Flashcards - Vocab Devo.
Polarization Problem Examples and Sets.
Polarization Problem Examples and Sets.
Quantitative Problem:
Quantitative Problem:
Unpolarised light of intensity I0 passes through a polariser and then through an analyser whose axis is rotated 30deg from that of the polariser. Determine the intensity of the light between polariser and analyser and the intensity of the light exiting the analyser in terms of I0.
Wicked Deep...
Multiple Choice Question (Ratios):
Multiple Choice Question (Ratios):
Two polarizers have polarizing axes that make an angle of 45˚ to each other. Unpolarized light of intensity I1 is incident on the first polarizer so that light of intensity I2 emerges from the second polarizer, as shown below. D
Polarization TSP Set
Polarization TSP Set
Everything below this is a work in progress or not applicable due to materials.
What is happening on an atomic / wavelength of light scale?
What additional questions do you have that you would like to answer? (Open Inquiry - IA)
5. Polarimeter - non-2021 Year
5. Polarimeter - non-2021 Year
The polarimeter is illuminated with 4 light emitting diodes at the bottom. The light from the LEDs is linear polarised by a polariser placed above them. The analyser at the top can be rotated and has a scale to measure angle in degrees as shown below. (Limited Inquiry)
Set the LED selector to green. Place a 100 ml measuring cylinder containing 100 ml of water into the polarimeter as shown below.
Place the analyser disk on top and rotate it until the green light is a minimum and read the angle using the pointer.
Now replace the measuring cylinder of water with one containing 100 ml of a sugar solution. Rotate the analyser again until the green light is a minimum and read the angle using the pointer.
What do you notice about the angle?
Sugar solutions are Optically Active. What do you think is meant by this? (Guided Inquiry)
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