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Polarized Light and its Interaction with Material

00:00:12;21 how we use polarized light or polarizing microscopes
00:00:17;19 for studying biological subjects,
00:00:21;17 especially living cells.
00:00:23;25 And polarizing microscopes
00:00:26;21 have been used by crystallographers
00:00:30;05 and mineralogists for many many years
00:00:33;09 for looking at crystals and natural minerals.
00:00:39;07 But what we need for biology
00:00:42;19 is slightly different from what the
00:00:45;20 standard commercial polarizing microscope is.
00:00:50;20 And therefore, I'm going to make some demonstrations here
00:00:56;27 to explain how the polarized light microscope works.
00:01:01;25 And these demonstrations partly use crystalline material
00:01:08;24 and partly biological examples.
00:01:11;21 So what we have here is the word "birefringence,"
00:01:19;06 I hope you can see on your screen,
00:01:22;05 and what I'm going to do is,
00:01:25;12 in front of this word,
00:01:27;25 I'm going to place a crystal of calcite
00:01:31;21 Calcite is Iceland spar,
00:01:35;04 and Iceland spar is a crystal calcium carbonate.
00:01:40;20 And it's well-known because it introduces
00:01:45;08 birefringence, or double refraction.
00:01:48;14 Double refraction is the property
00:01:51;17 that we use very often in biology.
00:01:54;08 And one of the properties of this double refraction
00:01:58;13 is that the light that comes through the crystal
00:02:05;05 before it's just a single image becomes double.
00:02:10;23 And because it's double, it's called birefringence.
00:02:14;08 And now if we put a pair of sunglasses in front of it.
00:02:20;07 one or the other of the image disappears.
00:02:23;29 What this is telling us is that light can come through
00:02:28;18 or the birefringent material itself is being polarized.
00:02:33;26 Now I'll explain polarization in a minute.
00:02:36;10 But in any event, birefringence,
00:02:39;18 or birefringent material takes ordinary light
00:02:46;08 and then splits it into two
00:02:48;16 and that split light itself becomes polarized.
00:02:54;11 So now what we're going to do
00:02:57;10 is take two birefringent materials,
00:03:00;22 two polarizing materials, here is polarizing material,
00:03:08;08 and putting it under the birefringent
00:03:09;29 Under the crystal, and then depending
00:03:16;07 on how I set the crystal,
00:03:17;29 we should be able to see two images.
00:03:22;18 Now then, when I put a second polarizing material above
00:03:31;27 and cross the two,
00:03:33;16 then we'll still see the birefringent material.
00:03:38;17 As I turn the crystal,
00:03:40;08 then the image becomes brighter or darker.
00:03:46;06 And this is the property of birefringence.
00:03:52;00 Calcite is not only interesting as a special mineral
00:03:56;26 it has strong birefringence,
00:03:59;21 but also it occurs in many of our body's structures.
00:04:04;04 And for example, in bone, and teeth, and so on,
00:04:07;26 one of the mineral components is calcite.
00:04:11;08 And when we look at a sea urchin embryo
00:04:15;26 for example, we see even in the young embryo
00:04:18;24 tiny little specules which are made of pure calcite
00:04:23;06 and those show as birefringence.
00:04:25;23 Instead of the calcite crystal
00:04:28;08 I'm going to put in a birefringent biological sample here
00:04:32;16 which you may just be able to see or not be able to see
00:04:38;16 because the contrast is terribly weak between cross polarizers.
00:04:43;09 Now I'm going to put another birefringent material
00:04:46;24 and it should become brighter or darker.
00:04:50;16 So this is a model of a mitotic spindle,
00:05:00;13 which is weakly birefringent
00:05:02;09 in fact the birefringence is only a few hundredths
00:05:08;06 or thousandths of the birefringence of the calcite crystal you saw.
00:05:12;17 So we need this compensator
00:05:15;27 in order to see which way the molecules are lined up.
00:05:19;26 But the birefringent material itself is just like the calcite crystal,
00:05:29;20 separating,depending on the orientation of the material.
00:05:36;25 What the birefringence is showing us
00:05:46;18 is that molecules are lined up and what I'm doing
00:05:51;05 is taking this piece of plastic and pulling it so that
00:05:55;17 the molecules line up.
00:05:58;12 It becomes bright and even shows color
00:06:04;13 and then this is birefringent just like the other material.
00:06:11;29 But let's just see here
00:06:14;03 lining up the molecule
00:06:15;20 made this birefringent material change
00:06:20;16 into a birefringent material.
00:06:23;00 I'll demonstrate the same effect by using sound waves.
00:06:27;06 Although sound is still somewhat different from light waves.
00:06:30;20 I cut a piece of wood here
00:06:34;01 so that one block is running parallel to the grain
00:06:39;04 and the other is across the grain.
00:06:41;06 And sound travels must faster along the grain
00:06:46;01 than across the grain,
00:06:47;12 so when I tap on the edge of this
00:06:50;13 then you'll find the resonance sound.
00:06:54;28 This is parallel to the grain (fast tapping sound),
00:06:57;13 and this is across the grain (slow tapping sound).
00:06:59;08 And you see there's almost an octave difference
00:07:04;06 because sound travels twice as fast
00:07:07;10 along the grain than across the grain.
00:07:09;17 So this is acoustic anisotropy,
00:07:12;26 somewhat similar to optical anisotropy.
00:07:17;14 Except there is a fundamental difference
00:07:21;11 between sound and light.
00:07:23;12 Sound travels faster in denser medium,
00:07:26;27 whereas light travels slower in denser medium.
00:07:30;23 So this distinction you should remember.
00:07:33;15 But anyhow, this is double refraction of a kind.

This Talk
Speaker: Shinya Inoue
Audience:
  • Researcher
Recorded: July 2011
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Shinya Inoue, one of the great innovators of polarization microscopy, describes what polarized light is and how light passing through a calcite crystal is split into two orthogonal polarized light beams.

Questions

Assessments

  1. When a long birefringent object (like a mitotic spindle) is placed between two crossed polarizers, at what angle will it appear brighter (for example, angle relative to the transmission axis of the first polarizer)?
  2. When Dr. Inoue stretches the plastic cellophane in-between the crossed polarizers, the image becomes brighter because
    1. The material absorbs light better
    2. The material becomes fluorescent
    3. The polymers in the material become more aligned
    4. The material becomes more transparent to light
  3. True or false. Both light and sound waves travel slower in denser medium.
  4. The term that describes the double refraction of light traveling through a crystal or other material is…..
  5. What is the evidence in this video that the images that appear through calcite are polarized in orthogonal directions?
  6. Research on your own. Why are there two images formed after light travels through a calcite crystal?

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Speaker Bio

Shinya Inoue

Shinya Inoue

Dr. Inoue is a Distinguished Scientist at the Marine Biological Laboratory in Woods Hole, MA. He has made numerous contributions to microscopy and cell biology, including developing polarized light microscopy and video microscopy. Inoue was awarded the Order of the Sacred Treasure by the Government of Japan and is a member of the National Academy… Continue Reading

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