Question

Which of the following is a dichroic crystal?
(A)Quartz
(B)Tourmaline
(C)Mica
(D)Selenite

Answer 1

Hint: The word dichroic means exhibiting or having two colors. Dichroism is exhibited by many minerals and by some organic compounds. Tourmaline is one of the best examples of such types of crystals.

Complete answer: In a dichroic crystal, the long chain molecules are arranged in a specific direction. They have the ability to completely absorb the vibrations of light parallel to these molecules. They do not obstruct the path of vibrations which are perpendicular to that specific direction. When ordinary light is passed through a thin slab of tourmaline crystal, the transmitted light is polarized. If we take two crystals parallel to each other, the light transmitted by the first crystal will also be transmitted by the second crystal. In case if we rotate the second crystal through a right angle, no light will be transmitted. This is because the absorption by tourmaline of all light rays vibrating in one plane not all the planes.
Therefore, option B is the correct answer.

Note:
Dichroic prism is an example of dichroic material. It is used in some camcorders which use several coatings to split light into red, blue and green components to record on separate CCD arrays. The property of some solutions and crystals of absorbing one of two plane polarized components of the transmitted light more strongly than the other is dichroism.