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Two polarising filters, one lamp. Turn one filter: where does the light go?
Sunglasses Test lab · Last updated: 16 October 2026
A lamp shines through two polarising filters, like the lenses in some sunglasses. You turn one filter and watch the light fade, then disappear, then return. Where does the light go?
What you'll discover
- Light waves can vibrate in many directions; a polarising filter lets only one direction through.
- Two filters at right angles block almost all the light.
- Glare reflected from water or roads is partly polarised, which polarised sunglasses block.
The science in plain words
Light is a wave, and in ordinary light from a lamp or the Sun the waves vibrate in all directions across the beam, up and down, side to side and everything in between. This is called unpolarised light.
A polarising filter contains long, lined-up molecules that let through only the part of the light vibrating in one direction. The light that comes out is polarised, and roughly half as bright as before.
Put a second filter after the first. If both are lined up the same way, the polarised light passes straight through. Turn the second filter and less light gets through. At a quarter turn, 90 degrees, the second filter blocks almost all of it and the view goes dark. Keep turning and the light comes back.
Light reflected off flat surfaces such as water, wet roads or car bonnets tends to be polarised horizontally. Polarised sunglasses have filters that block horizontal vibrations, so they cut that glare while letting other light through. Polarisation is also used in LCD screens, in some 3D cinema glasses and by engineers to find stress in plastic parts. Bees use the polarisation of skylight to navigate.
Where it fits
- India, NCERT: Polarisation goes beyond the NCERT syllabus for classes 6 to 10; it builds on light in Class 10 Chapter 9: Light: Reflection and Refraction.
- United States: Grades 9 to 12
Common questions
How do polarised sunglasses work?
Light reflected from water or roads is mostly vibrating horizontally. Polarised lenses only let through light vibrating vertically, so they block much of that glare while you still see clearly.
What happens when two polarising filters are crossed?
The first filter lets through light vibrating in one direction. If the second is turned at right angles to it, that light cannot pass, so almost nothing gets through and it looks dark.
What is polarised light?
It is light whose waves all vibrate in the same direction across the beam. Ordinary light from the Sun or a bulb vibrates in every direction and is called unpolarised.
Related labs
- Bending Light: Shine a ray into a glass block. How much does it bend?
- Rainbow and Beyond: Slide from ultraviolet through the rainbow to infrared. Which light makes the star glow?
- Prism Rainbow: Split white light into a rainbow with a glass prism.
See all physics labs or every Taz science lab.
Try it yourself
The Sunglasses Test lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.