Looking at Ice Crystals
Scientists on the SIPEX voyage are taking sea ice cores and slicing them very thinly, lengthwise with a band saw so that they can look at the ice crystals which make up the ice. They have to do this in a lab that’s kept at -20°C so the ice doesn’t melt. They analyse the ice crystals to get more information about how the ice is formed at different places, in different conditions and other physical properties the ice has.
Looking at ice crystals isn’t as easy as it sounds as it’s practically impossible to see them in ordinary light. The scientists use a combination of polarising films to see the edges of different crystals. To understand how this works you need to know a little bit about light, refraction and how polarisers work.
If you already know a bit about any of the above you can jump to different sections using the links below:
Light
Refraction
Polarisation
How do refraction and polarisation help scientists to see ice crystals?
Light
Light is part of the electromagnetic spectrum (see 'Understanding Altimetry') which means it is an electromagnetic wave. Electromagnetic waves are made up of an oscillating (moving up and down) electric field, E, coupled with an oscillating magnetic field, B, at right angles to one another.
Oscillating electric and magnetic fields making up an electromagnetic wave
These waves do not exist independently. In other words, they always occur together; therefore there is only one electromagnetic wave.
Usually when thinking of electromagnetic waves we use the electric field, E, to describe the plane (orientation) on which the wave is travelling. This makes it easier for us to draw and describe; we just have to remember that there is a magnetic field there as well.
Although light is made up of waves, we often draw it as a ray. A ray diagram uses a straight line with an arrow on the end to show which direction the light wave is travelling in. When light hits a surface, 3 main things can happen, it can be absorbed, reflected or refracted or a combination of all three.
Ray diagram showing absorption, reflection and refraction of light at a surface
Refraction
When a wave travels across a boundary between two different substances (media) e.g. air and ice, its speed changes. For example, the speed of light in air is 3×108m/s, whereas in ice it’s slowed down to a speed of around 2×108m/s. The change in speed can result in a change in direction of the light waves. This change in direction is called refraction.
Refraction of light at an air-ice boundary
How much the ray of light is ‘bent’ by being slowed down or speeded up as it passes through different media is dependent on the refractive index of the medium. Refractive index, 1n2, is simply a ratio of the velocities of light in different substances and is defined as
| Substance | Refractive Index, n |
| Air | 1.000 |
| Water | 1.333 |
| Ice | 1.310 |
| Salt | 1.544 |
| Diamond | 2.417 |
The higher the refractive index of a substance the more it slows the light down, which in turn makes it change direction more dramatically.
Polarisation
The light that you see all around you, from the sun and from light bulbs etc, is made up of many electromagnetic waves all travelling independently in different directions. This is called unpolarised light.
Light can be polarised by passing it through a polariser. A polariser is a material that will only allow light waves orientated in one direction to pass through. When all the light waves are orientated the same way the light is said to be polarised.
Unpolarised light being filtered by a polariser
As a polariser only lets waves in a particular plane pass through, if you put two polarisers together at right angles to one another you can block the electromagnetic (light) waves altogether. You can see in the diagram below that the light waves strike the first polariser, and only those aligned vertically can pass through. These vertical waves then hit the second polariser which only allows waves aligned horizontally to pass through. There are no horizontal waves as they were filtered out by the first polariser, therefore no light passes through the second polariser.
Light can be stopped altogether by placing two polarisers at 90o to one another
How do refraction and polarisation help scientists to see ice crystals?
In order to see the crystals within a thin section of ice, the scientists place the slice of ice core between 2 polarisers and light it from underneath, as shown in the diagram below.
Light's journey through polarisers and ice before producing an image of the ice crystals present
Unpolarised light passes through a polariser so the scientists know that all the light waves hitting the ice are aligned in the same way. The polarised light then hits the ice crystals. As all the crystals have a slightly different structure and orientation, the polarised light gets refracted differently by each crystal. Therefore the emerging light is orientated in different directions depending on which crystal it passed through and in what way the light was slowed down. This light then passes through the 2nd polariser which will block any light orientated in the same direction as the 1st polariser but will let through any light that has been reorientated in its direction. Any light in between these two orientations will partially make it through the second polariser but with less intensity. This means that the image the scientists see through the 2nd polariser shows areas of different shades corresponding to different ice crystals.
