Index of Refraction Motivation Observation Skills

Light travels at different speeds through different substances. The reason we can see a clear colorless piece of glass submerged in a clear colorless liquid such as water is because the light gets slowed down and refracted (bent) as it passes from the water to the glass. The Ghost Crystal is made almost all of water so the light rays are bent about the same making the crystal invisible in the water.

Ghost Crystals (prepare ahead)
String to tie crystal ( 1-2 mm diameter)
Beaker
Deionized Water

Place 5-10 of the Ghost crystals in a large, clean glass jar/beaker and fill with deionized water to within 3 cm of the top. Place lid on jar and wait a few hours. When the crystals have grown to the point where they appear essentially invisible, pour out 4 or 5 of them into a clean petri dish. Although they appear like chunks of glass, they actually are quite rubbery like jell-O.

Choose one of the crystals, and touching it as little as possible, tie a string around it snugly, but not so tight that it cleaves the crystal in half. This will take some practice. Fill the jar with deionized water, pick the crystal up by the string and lower it into the jar. The crystal will seem to disappear!

Safety

Wear protective gloves and safety goggles.

Have students describe what they observe in a beaker. They will see a noose with apparently nothing in it in the jar. Lift up string and crystal will now be visible because the index of refraction of water (the Ghost Crystal) and that of the air its now surrounded by are quite different!

1) Because oils from your skin can get absorbed onto the crystal and cloud its surface, wash your hands throughly and even then, handle the crystal as little as possible.

2) If bubbles form inside the crystals, try placing the jar in the refrigerator for a few hours to dissolve the excess gas (gases dissolve more in cold water than warm).