A Polarised Light Experiment With Two Sunglasses

Hold one pair of polarised sunglasses up to your eye. Hold a second pair in front of it. Now slowly rotate the second pair. At 90° you cannot see through them at all: not dim, not tinted, essentially black. Two pieces of glass that each let plenty of light through, stacked, block almost all of it. This polarised light experiment takes about ninety seconds to set up and it is the cleanest household demonstration that light is a wave with a direction, not just a stream of brightness. It also contains a nice trap: the explanation nearly every textbook gives, about slots in a picket fence, gets the physics backwards.

Key Takeaways

• You cross two polarised lenses, rotate one, and watch transmission fall to almost zero at 90°, which follows Malus’s law I = I₀·cos²θ.
• Light is a transverse wave. Each lens passes only the component of the electric field along its transmission axis, so rotating one lens changes what survives.
• Check your sunglasses are genuinely polarised first by looking at an LCD screen and rotating them. Plenty of cheap “polarised” sunglasses are just tinted.
• Best for ages 9 to 15. Younger children enjoy the blackout; the wave model needs some abstraction.
• What usually goes wrong: one of the pairs is not polarised, so nothing happens at any angle.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Polarised sunglasses	2 pairs, 3 is better	Household, or a chemist’s cheap fishing pair	Two sheets of polarising film, sold for school science for the price of a sandwich
LCD computer or laptop screen	1	n/a	A supermarket self-checkout screen works; so does a calculator display
Clear plastic items	3–4	A CD case, a disposable fork, a clear ruler, a plastic bottle	Anything transparent and moulded
Sticky tape	1 roll	n/a	Overlapping layers on a glass slide give the best colours
Glass of water	1	n/a	A dark-coloured tray of water works better for the reflection test
Protractor	1	n/a	Mark 0°, 45° and 90° on a card and hold the lens against it
Torch or lamp	1	n/a	A window with daylight, never pointed at the sun

Time: prep 5 min, experiment 25 min, cleanup none Cost: nothing if you own two pairs of polarised sunglasses Mess level: low

Do the Experiment: The Polarised Light Experiment

Step 1 - Verify both pairs are actually polarised

Look at a white area of an LCD laptop or monitor screen through one lens and rotate the glasses slowly through 90°. A genuinely polarised lens will go noticeably dark at one orientation. If nothing changes, the glasses are tinted, not polarised, and the rest of the experiment cannot work. Test both pairs.

Step 2 - Cross them and ask for a prediction

Hold the two pairs in front of each other with their arms parallel, so the lens axes line up. Ask: “If I turn one pair sideways, will it get darker, brighter, or stay the same? And how much darker?” Most people predict “a bit darker.” Write it down.

Step 3 - Rotate slowly and watch it go out

Keeping both pairs in line with your eye and a bright window or lamp, rotate one pair gradually to 45°, then to 90°.

What you should see: At 0° you see through both as if through slightly darker sunglasses. By 45° it is noticeably dimmer. At 90° the view is essentially black, dark enough that you cannot read large print through it. Not dim. Black.

Step 4 - Measure the halfway point

Set the top pair at 45° against a protractor and compare it to 0° by eye. Malus’s law says transmission at 45° should be cos²45° = 0.50 of the 0° value, which is a one-stop drop in photographic terms.

What you should see: A clear dimming but a perfectly usable view at 45°, which is a good match to the prediction of half. The lesson is that the fall-off is not linear in angle; nothing much happens for the first 20°, then it collapses between 70° and 90°.

Step 5 - Find the glare and kill it

Fill a dark tray or a glass with water and look at the surface at a shallow angle, around 35–40° above horizontal. Look through one lens and rotate it.

What you should see: The reflection off the water nearly disappears at one orientation and returns at 90° to it. The reflected light is partially polarised in the horizontal plane, which is exactly what polarised sunglasses are designed to block.

Step 6 - Squeeze plastic between crossed lenses

Set the two pairs at 90° so the view is black, then slide a clear plastic fork, a CD case or several overlapping layers of sticky tape into the gap.

What you should see: Bands of colour, often brilliant, strongest where the plastic has been bent, moulded or stressed. The dark field lights up in rainbows that follow the stress pattern. This is how engineers find stress concentrations in transparent models.

Step 7 - Try your phone and note the difference

Repeat Step 1 on a phone screen rather than a laptop.

What you should see: On many phones the darkening is weaker and the screen may shift colour instead of simply going dark. Most modern OLED phones carry a circular polariser for anti-reflection, which behaves differently from the plain linear polariser in a laptop’s LCD stack. Noticing that difference is a real observation, not a fault.

What’s Actually Going On

Light is an electromagnetic wave, and its electric field oscillates perpendicular to the direction the wave travels. Sunlight is unpolarised: the oscillation direction is randomly distributed across all angles in that perpendicular plane. A polariser transmits only the component of the electric field lying along one particular direction, its transmission axis, and absorbs the rest. Pass unpolarised light through one ideal polariser and you keep about half of it; everything that emerges now oscillates in a single plane.

Put a second polariser in the path at an angle θ to the first, and the surviving wave is projected onto the new axis. Because intensity goes as the square of the field amplitude, the transmitted intensity is I = I₀·cos²θ, which is Malus’s law. At θ = 0 it is 1. At 45° it is 0.50. At 90° it is exactly zero for ideal polarisers, and perhaps 0.1% for real sunglasses, which is why crossed lenses look black rather than merely dark.

Now the correction, which is the most interesting part. The standard analogy is a picket fence or a slotted grating: the wave “fits through” slots aligned with its oscillation. That picture is appealing and it gets the microscopic mechanism backwards for the sheet polarisers in your sunglasses. Edwin Land’s H-sheet, described in his 1951 paper in the Journal of the Optical Society of America, is polyvinyl alcohol stretched to align its long polymer chains and then doped with iodine so the chains conduct along their length. An electric field oscillating along the chains drives electrons back and forth, loses energy and is absorbed. An electric field across the chains has nowhere to push and passes through. So the transmission axis is perpendicular to the molecular chains, which is the opposite of the fence intuition. If you want a mechanical analogy, think of absorption by a conductor, not geometry by a gap.

Two more pieces of real physics. Reflection polarises light: at Brewster’s angle, which for air to water is arctan(1.333) = 53.1° from the normal, the reflected light is completely polarised parallel to the surface. That is why polarised sunglasses have their transmission axis vertical, to reject horizontally polarised glare off roads, water and car bonnets. And many clear plastics are birefringent when stressed, meaning their refractive index differs along two axes, so one component of the wave is retarded relative to the other and the recombined light emerges with its polarisation rotated by an amount that depends on wavelength. Different colours rotate by different amounts, which is why the stressed fork shows rainbows rather than grey.

How to Explain It to Your Kid

Ages 5–8: The lens only lets one wiggle direction through

Have them hold a skipping rope and wiggle it up and down, then side to side, so they feel that a wave has a direction. Say that light wiggles too, and the lens only lets through the wiggles going one way. Then let them do the blackout themselves and turn the glasses back and forth. The only idea to land is that light has a direction you cannot see until you use the right filter.

Ages 9–12: cos² and why 45° is halfway

Introduce polarisation and the transmission axis. Then give them the numbers: 0° passes everything, 90° passes nothing, 45° passes half. Ask them to guess 30° and 60° before you tell them the answers are 0.75 and 0.25. That asymmetry, where equal steps in angle give unequal steps in brightness, is a good first encounter with a non-linear relationship, and it pairs well with how touchscreens use the layers above an LCD.

Ages 13+: The three-polariser result

Cross two lenses so the field is black. Now slide a third lens in between them at 45°. Light comes back. Adding a filter increases transmission, which looks impossible and is not: each polariser projects the field onto a new axis rather than merely blocking what does not match. Have them compute the result, 0.5 × cos²45° × cos²45° = 12.5% of the original unpolarised intensity, and measure it with a phone light meter app. This is also the classical analogue of a sequence of quantum measurements, which is worth mentioning but not overselling.

The question to ask: “If polarised sunglasses block glare off a wet road, what would happen to that glare if you tilted your head 90° to one side? Predict it, then try it the next time it rains.”

Why Teaching This Early Pays Off

Polarisation is the first property of light that a child cannot detect with their eyes alone, which makes it an unusually clean introduction to an idea at the centre of science: there are real, measurable properties of the world that human senses simply do not register, and instruments extend what counts as observable. That framing pays off later in magnetism, ultraviolet, infrared and radio.

There is also a concrete curriculum payoff. Transverse versus longitudinal waves is a standard exam distinction, and students routinely lose marks on it because they have only ever seen it as a definition. Sound cannot be polarised, because it is a longitudinal pressure wave with no transverse direction to filter. Light can. A child who has crossed two lenses has a physical reason to remember which is which.

Galili and Hazan’s 2000 analysis of learners’ knowledge in optics found that students’ optics models are built from fragments that coexist inconsistently rather than one coherent wrong theory. This matters for how you run the activity. Do not try to replace their whole picture of light in one afternoon. Target one specific fragment, which here is “a darker filter just dims things”, and give it a result it cannot explain.

Before You Start: What Parents Should Know

Best age	9 to 15. A five-year-old enjoys the blackout but will not retain the wave model
Supervision	Minimal. The only real issue is borrowing someone’s good sunglasses
Real hazards	None, with one firm exception: never use any of these filters to look at the sun. Crossed polarisers cut visible light but do not reliably block infrared, and no combination of household lenses makes solar viewing safe. If you pop a lens out of a frame, the plastic edge can be sharp and the lens can crack
Don’t	Don’t substitute 3D cinema glasses for the measurement steps. Most are circular polarisers, so crossing two gives a muddy result that does not follow cos²θ. They are fine for the stress-colour step

When It Doesn’t Work

• Nothing changes at any angle. At least one pair is tinted rather than polarised. Run the LCD test from Step 1 on each pair separately. This is by far the most common failure.
• It gets dim but never black. Either the lenses are low-quality polarisers, or you are not reaching a true 90°. Also check you are not looking at light leaking around the frames rather than through both lenses.
• The water-glare step shows nothing. Your viewing angle is too steep. Get your eye down near the water’s level and look across the surface at roughly 35–40° above horizontal, and use a dark tray so you are not seeing the bottom.
• The plastic shows grey, not colour. Unstressed plastic gives little effect. Use a moulded item, bend a clear ruler gently while watching, or build up four or five overlapping layers of sticky tape on a glass slide.
• Your phone test gives a strange colour shift. Expected on OLED phones with circular polarisers. Use a laptop LCD for the clean result and treat the phone as a bonus observation.

Take It Further

• Plot Malus’s law. Tape one lens flat over a phone’s light sensor, mount the second on a protractor, and record a free light-meter app’s reading every 15° from 0° to 90°. Plot reading against cos²θ. A straight line is the result, and a straight line through nine points is a strong science-fair graph.
• Map the polarised sky. On a clear day, with your back to the sun, look through one lens at a patch of sky 90° away from the sun and rotate. The sky dims and brightens by roughly a quarter. Compare that to a patch near the sun, where the effect is far weaker. This is the same scattering physics behind why the sky is blue.
• Build a stress viewer. Tape one lens to a torch and hold the other above it to make a permanent crossed-polariser station, then test clear plastic objects for stress patterns. Compare a brand-new plastic fork to one you have deliberately bent. Then look at a prism or a clear cube, which brings in the dispersion from making a rainbow indoors with a prism.

What to Watch For Over the Next 3 Months

• Week 4: They tilt their head in the car to see what happens to windscreen glare, or notice the rainbow patches on a car’s rear window seen through polarised glasses.
• Month 2: The word “polarised” starts getting used correctly, and they ask whether sound or water waves can be polarised. The answer, no for sound and yes in a limited sense for surface waves, is a good conversation.
• Month 3: If they ask how a liquid crystal display uses this to make a pixel go dark, they are ready for the actual answer, which is that the liquid crystal layer rotates the polarisation under an applied voltage and sits between two crossed polarisers.

Frequently Asked Questions

How can I tell if my sunglasses are really polarised?

Look at a white area of an LCD laptop or monitor screen and rotate the glasses through 90°. A polarised lens goes clearly dark at one orientation. A merely tinted lens looks the same all the way round. This test takes five seconds and is more reliable than the label.

Why do crossed lenses go completely black rather than just dark?

Because transmission follows cos²θ, which is exactly zero at 90° for ideal polarisers. Real sunglasses leak a fraction of a per cent, which your eye reads as black. The surprise comes from expecting the two tints to add up rather than one filtering what the other left.

Do polarised sunglasses actually help, or is it marketing?

They genuinely cut reflected glare, because light reflected off a horizontal surface at a shallow angle is partially polarised horizontally and the lens axis is vertical. The effect is strongest on wet roads and water. They do nothing extra for ultraviolet protection, which depends on the lens coating and material.

Why do some car and plane windows look blotchy through them?

Tempered and laminated glass is stressed, and stressed material is birefringent, so it rotates polarisation by amounts that vary with wavelength and position. You are seeing the stress pattern frozen into the glass, the same effect as the plastic fork in Step 6.

Can I do this with 3D cinema glasses?

For the stress-colour step, yes. For the Malus’s law measurement, no. Modern 3D glasses use circular polarisers, which do not cross to black and do not follow cos²θ. Plain linear polarising film is cheap and gives clean results.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Land, E. H. (1951). “Some Aspects of the Development of Sheet Polarizers.” Journal of the Optical Society of America, 41(12), 957. https://doi.org/10.1364/JOSA.41.000957
2. Georgia State University. “Polarization by Absorption: Dichroism.” HyperPhysics. https://hyperphysics.gsu.edu/hbase/phyopt/polabs.html
3. Georgia State University. “Classification of Polarization.” HyperPhysics. https://hyperphysics.gsu.edu/hbase/phyopt/polclas.html
4. American Academy of Ophthalmology. “How to Choose the Best Sunglasses to Avoid Sun Damage.” https://www.aao.org/eye-health/glasses-contacts/sunglasses-3
5. Galili, I., & Hazan, A. (2000). “Learners’ knowledge in optics: interpretation, structure and analysis.” International Journal of Science Education, 22(1), 57–88. https://doi.org/10.1080/095006900290000
6. NASA Science. “Introduction to the Electromagnetic Spectrum.” https://science.nasa.gov/ems/01_intro/
7. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165