Purpose:

Smartphone Screen Polarization: To explore light polarization and the LCD technology used to illuminate cell phone screens.

Optional Sensor App Activity Outline: To collect and analyze smartphone sensor data through a selected app.

Procedural overview: Students can complete a few simple light polarization exercises to model the LCD technology used to illuminate smartphone screens. Then students can choose an app that collects sensor data and demonstrate how the app analyzes and utilizes the data to perform a specific function.  

Approximate class time: One class period.

Smartphone screen polarization

Materials:

• Smartphone
• Optical polarizer sheets
• Clear plastic fork or spoon
• Small flashlight
• Clear plastic CD cases, DVD cases, bottles and product packaging

Background information: Light is electromagnetic radiation that travels through space at a specific speed. Both an electric and magnetic fields oscillate perpendicular to each other, relative to the light’s direction of travel. Which way the field moves is called the direction of polarization. Usually light is unpolarized, or a mixture of both polarizations.

An optical polarizer is a thin sheet of plastic in which all of the long plastic molecules are lined up in the same direction. Only light waves with a specific polarization pass through an optical polarizer and it blocks light of other polarizations. An optical polarizer converts light of mixed polarizations into a single direction of polarization.

Student questions and answers:

1. Hold a polarizer flat over a flashlight beam. Rotate the polarizer. What happens and why? Is the light coming from the flashlight polarized or unpolarized?

The overall brightness fluctuates a little bit as the polarizer is rotated. Light shows through the polarizer no matter the angle of the polarizer, meaning the light is likely unpolarized.

2. Hold one polarizer over a flashlight. Take a second polarizer and slowly rotate it, so it is eventually perpendicular to the other one. What happens and why?

The overall brightness changes as the polarizer is rotated. Eventually, all of the light is blocked, since one polarizer blocks light of one polarization, and the other blocks light of the other polarization.

3. With your smartphone screen illuminated, hold a polarizer flat over the screen. Rotate the polarizer. What happens and why? Is the light coming from the LCD polarized or unpolarized?

The overall brightness changes as the polarizer is rotated, and at some angles, almost all of the light will be filtered out by the polarizer. Since one of the last items that the light travels in an LCD is a polarizing filter, the light LCDs display is polarized. If the polarizing filter is aligned exactly perpendicular to the LCD polarized light, it should prevent the polarized light from transmitting through the filter.

4. Hold one polarizer over a flashlight, and place a clear plastic utensil on top of the first polarizer. Hold a second polarizer over the utensil, and slowly rotate the top polarizer. Then, slowly rotate the fork or spoon. What happens in each instance and why?

In all cases, you see rainbows in some areas of the clear plastic. In areas where the plastic was stressed, the plastic molecules are bent, and they bend the direction of polarization of light, allowing some light that has passed through one polarizer to have the correct polarization to pass through the other. Different wavelengths of light have their polarization bent by different amounts, so you see a spectrum of colors.

5. Take away the polarizer sitting on top of the plastic utensil. Can you see the same effect with only one polarizer below the utensil? Why?

No. The light from the flashlight is unpolarized. To get the rainbow effect, there must be something on both sides of the fork/spoon that polarizes light.

6. Place the plastic utensil directly on top of an illuminated smartphone screen and hold one polarizer above the smartphone screen. Can you see the same effect? Why?

Yes. The light from the smartphone screen on one side of the clear plastic fork/spoon is polarized, and the polarizer is on the other side, so you get the rainbow effect.

7. Obtain other plastic items from your instructor and predict where the stress patterns are for those items. Use a viable method (one that you learned from above) to test your predictions. Explain whether your predictions were accurate and what testing method you used.

Students should use the method outlined in numbers 4 or 6 above. Other predictions and observations will vary.

8. What does this experiment demonstrate or model about how smartphone screens work?

The pixels in a smartphone LCD screen have two perpendicular polarizers with liquid crystal in between. Depending on how much electric field is applied to that pixel of liquid crystal, its molecules align to rotate light polarization by various amounts, making the pixel appear to have the desired brightness. A clear plastic item between two polarizers or between one polarizer and the polarized screen acts like the liquid crystal.

Optional sensor app activity suggestions

Materials:

• Students’ smartphones
• Other supplies as needed

Notes to the teacher: There is a wide range of available apps that use the various sensors in smartphones for scientific experiments. There are also tools that teach app creation. If you would like your students to explore some of these apps directly, allow time for them to find an app that interests them.

If smartphone resources are available for each student, students can find a smartphone app that uses and shows sensor data, download the app, learn how it works based on the sensor data it collects and explain and demonstrate the app to their classmates. This is a very open-ended list of suggested apps that use sensor data.

The list below gives examples of a variety of scientific apps, with links for students to create their own apps if they would like (using MIT App Inventor). Students might search and find additional scientific apps as well.

Potential apps separated by sensor technology:

Seismometer

                MyShake seismometer app, UC Berkeley

Holographic display

                Instructables: Turn your smartphone or tablet into a 3-D hologram

Image projector

                Liliputing: Make your own smartphone projector with a shoebox

Microscope

Amazon: 400x microscope attachment for smartphone cameras

Amazon: 60x microscope attachment for smartphone cameras

Spectroscope

               AlphaNov, Gospectro: The power of spectroscopy at your fingertips

               ChemEdXchange: Use your smartphone as an absorption spectrophotometer

Thermal imager

               FLIR ONE: Thermal imaging camera attachment

Radiation detector

Hackaday: Turn your camera phone into a Geiger counter

Idaho National Laboratory: Smartphone-based radiation warning system

Audio spectrum analyzer

               Channel Pro Network: Best audio spectrum analysis apps for iOS and Android

Magnetic field sensor

Google: Metal detector app

DNA polymerase chain reaction

               Minipcr: Apps to control thermocycler for DNA polymerase chain reaction

Miscellaneous ideas

Ditch That Textbook: 11 class activities with sensors you didn’t know your phone had

Google: Mobile sensor apps for learning

Elsevier: New ways to use smartphones for science

Scientific American: 8 apps that turn citizens into scientists

Science on Stage: Smartphones in science teaching

Scizzle: 5 killer ways to use your smartphone for science

Make your own science app

MIT: App inventor

Teach Engineering: Storing Android accelerometer data: app design

Lab4U, Lab4Physics and Lab4Chemistry apps

Smithsonian magazine’s Lab4U puts a science lab in your pocket