Describe the visual effect of placing a piece clear Iceland Spar on top word geology in text book?

Placing a piece of clear Iceland spar on top of the word "geology" in a text book would cause the text to appear to be doubled. This is because Iceland spar is a birefringent material, meaning that it has the property of splitting a light wave into two rays. The two rays are known as the ordinary ray and the extraordinary ray. The ordinary ray travels at the same speed as light in air, while the extraordinary ray travels at a slower speed. This difference in speed causes the two rays to be refracted at different angles when they pass from air into Iceland spar. The result is that the text appears to be doubled, with one image displaced slightly from the other.

The amount of displacement depends on the thickness of the Iceland spar and the wavelength of the light. For visible light, the displacement is typically about 0.5 mm. This is small enough that the two images can be seen as a single image, but it is large enough to be noticeable.