Laser Demonstration of Calcite

Purpose of Demonstration: To demonstrate a gemstone's ability to split a single beam of light into two or more beams. This demonstration graphically shows the birefringence of a gemstone, most notably calcite which has a birefringence of .172 which is extremely high. Meaning that the gemstone will split a single beam of light into two beams, which, due to the clarity and large size of available crystals, allows us to demonstrate this splitting of a light beam using a laser. It is this ability of gemstones to split a beam of light into two or more beams that allows us to have gemstones such as tanzanite, andalusite, and iolite to name a few. These gemstones offer remarkably different colors in different directions of observation using a dichroscope. But the separations of the beams is so small that our eyes cannot see this separation without a dichroscope. Fortunately, the separation in calcite is so strong, we are able to see this optical property in the following demonstration.

Equipment Required: A fairly clean (clear) calcite crystal of at least 2x2 inches in length/width. And a regular laser pointed available from any Office Max or Office Depot...and probably half the students in class.

Suggested Grade Level: Middle School and up to gemological teaching level.

Degree of Difficulty: Easy.

Cautions: Don't point the laser into anyone's eyes. Even the low power laser pointers can cause damage to young eyes.

Here's how it works: Simply turn out the lights and shoot the laser beam into the calcite crystal from varying directions. Some crystals are oriented so that the long direction will show the best separation. But it is just a matter of pointed the laser into the stone and finding the best direction of observation. The laser light beam will split into two distinct beams with occasional "X" patterns showing.

Here is what you should see (note that fuzziness of photos is due to light viewed through calcite):

Photo#1

Shows the laser light source (#1) with the beam of light traveling through the stone and splitting into two beams. One travels toward the side of the calcite crystal (#2) and the other exits the crystal at the top (#3). This is a fairly clean crystal that I have used for demonstration and photography purposes here. For classroom purposes it is not required to use such a high clarity crystal (just in case you cannot find one)

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Photo #2

This is a second view of the same calcite crystal with a single laser beam. It also demonstrates (#1) laser light source, (#2) light beam number one traveling out the side of the crystal, and (#3) light beam number two traveling out top of crystal. Due to the nature of the laser the individual light beams are very dramatic in person.

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Photo #3

This photo shows the "X" pattern that you may see looking through the short side direction of the calcite crystal. This pattern is formed when the light beams are in transition and reflecting off the opposing side of the calcite crystal.

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Photo #4

This view is looking diagonal into the crystal with the laser beam coming through the bottom of the stone. You will notice two distinct light beams coming through the stone, and the "X" pattern reflected off the top of the stone. Remember, these light reactions are being generated by a single laser beam of light.

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What you should not be seeing:

Photos #5 & #6

These photos show a calcite crystal that is too included. The light beams are dispersed due to many inclusions in the crystal which make the separation of the light beam very difficult to see. Be sure that you obtain the cleanest crystal possible. For more information contact YourGemologist.

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