How do polarized sunglasses reduce glare?

1. It is a sunny day at the lake. Sunlight bounces off the water straight into Mia's eyes. That bright shine is called glare, and it hides the fish.
2. Light travels as a wave, and waves wiggle. Sunlight is a mix of waves wiggling in every direction: up and down, side to side and everything in between.
3. When the light bounces off the flat water, something changes. The side-to-side wiggles bounce best, so the glare wiggles mostly side to side.
4. A polarized lens works like a tiny fence for light. Up-and-down wiggles get through the gaps. Side-to-side wiggles, like the glare, are stopped.
5. Mia puts on polarized sunglasses. The side-to-side glare is blocked, but light from the fish still gets through. Now she can see under the water!
6. Here is the proof. Turn a lens a quarter turn and its gaps lie flat, so the glare gets through again. Turn it back and the glare disappears.

Tap each part of the sunglasses and the light. Watch what moves!

• Frame. The frame holds both lenses upright. That matters: a polarized lens only blocks glare when it is the right way around.
• Lens. The lens is a sandwich: tough clear layers outside and a very thin special film in the middle. A dark tint dims all the light a little.
• Polarizing film. The film is made of long, thin chains that were stretched until they all lined up. The chains soak up light that wiggles along them, which is the side-to-side glare.
• Pass direction. Every polarized lens has a pass direction. In sunglasses it points up and down, so up-and-down wiggles get through.
• Glare from the water. Glare from flat water wiggles side to side. That is across the pass direction, so the lens soaks it up and turns it into a tiny bit of warmth.
• Light from the fish. Light from the fish wiggles every way. The up-and-down part gets through the lens, so you still see the fish, only a little dimmer.

Turn the filter by dragging its knob or using the slider. Compare the water inside the filter with the water outside. When is the glare weakest?

Experiment: The filter's arrow lies flat and the glare shines through. What happens when you give the filter a quarter turn?

1. Start with the green arrow lying flat. Look at the water inside the filter.
2. Slowly turn the filter until the arrow stands upright.
3. Press Record at a few angles to compare.

What we found: The glare wiggles side to side. When the filter's arrow lies flat, the glare gets through. When the arrow stands upright, the glare is blocked and you can see the fish. In between, some of it gets through.

Turn on JavaScript to try it yourself.

You are a nature photographer. For each of the three photos, turn the filter until the glare is gone, then take the picture.

Light travels as a wave. Like a wave on a rope, it wiggles as it goes. Light from the Sun or a lamp is a mix of waves that wiggle in every direction: up and down, side to side and every slant in between.

When this light bounces off something flat and shiny, such as a lake, a wet road or a window, the mix changes. Wiggles that run along the surface bounce off best. So the glare from flat water wiggles mostly side to side. Scientists say the light has become polarized, which means its wiggles are lined up.

A polarized lens contains a very thin film that lets through only one direction of wiggle. In sunglasses, that direction is up and down. The side-to-side glare is soaked up by the film, while about half of the ordinary light from everything else still gets through. That is why the glare disappears and you can suddenly see fish under the water.

The picket fence is a helpful picture, but it is not exactly how the film works. The real film is made of long chains that soak up light wiggling along them. So the real chains lie flat, across the direction that gets through. Also, polarized lenses cannot remove all glare: light bouncing off metal, or glare from an upright window, is not lined up the same way.

Key words

Glare

Very bright light bouncing off a shiny surface into your eyes.

Polarized light

Light whose wiggles are all lined up in one direction.

Polarizing filter

A sheet that lets through only one direction of wiggle.

Transmission axis

The direction of wiggle that a polarizing filter lets through.

Malus's law

The brightness that passes a polarizer equals the incoming brightness times the cosine squared of the angle between the light's vibration and the filter's axis.

Brewster's angle

The angle at which light reflected from a surface is completely polarized.

Check your thinking

1. Which light do polarized sunglasses block best?

   Show the answer

   Glare that wiggles side to side. Glare from flat water wiggles side to side, across the lens's pass direction, so it is blocked.

2. You hold polarized sunglasses in front of a lake and turn them a quarter turn. What happens?

   Show the answer

   The glare gets through again. After a quarter turn the pass direction lies flat, the same way the glare wiggles, so the glare passes.

3. Why can you still see the fish through polarized sunglasses?

   Show the answer

   Their light wiggles every way, so part of it gets through. Light from the fish is a mix of all wiggles. The up-and-down part passes the lens.

4. Polarized glare hits a filter whose axis is at 60 degrees to the glare's vibration. How much gets through?

   Show the answer

   One quarter. The cosine of 60 degrees is one half, and one half squared is one quarter.

5. Sunlight shines off an upright shop window. Which way does that glare mostly wiggle?

   Show the answer

   Up and down, along the window. Glare wiggles along the surface it bounced off. An upright window makes up-and-down glare, which normal polarized sunglasses let through.

6. You put one polarized lens on top of another and turn the top one a quarter turn. What do you see?

   Show the answer

   It goes dark, because almost no light passes both. The first lens lets through only one wiggle. The second lens, turned a quarter turn, blocks exactly that wiggle.