Question

Choose the CORRECT pair of crystal systems that represents the optic orientation shown in the figure. (X, Y, Z are the principal optical vibration directions and a, b, c are the crystallographic axes). 

• A. Orthorhombic – Triclinic
• B. Orthorhombic – Monoclinic
• C. Cubic – Hexagonal
• D. Tetragonal – Monoclinic

Hint:

Orthorhombic: X‖a, Y‖b, Z‖c.
Monoclinic: Y‖b, X and Z inclined to a–c plane.

Answer 1

The Correct Option is B

Step 1: Understanding optic orientation.
In anisotropic crystals, the vibration directions (X, Y, Z) of light correspond to the optical indicatrix axes. These are related to the crystallographic axes (a, b, c) differently depending on symmetry.
Step 2: Identifying the orientation.
- For orthorhombic crystals, the optical vibration directions (X, Y, Z) coincide with the crystallographic axes (a, b, c).
- For monoclinic crystals, the Y (optic) direction corresponds to the crystallographic axis \( b \), but X and Z are oblique to a and c.
Step 3: Analyzing the figure.
The left diagram shows all optic axes aligned with crystallographic axes → Orthorhombic. The right diagram shows Y coinciding with \( b \)-axis, but X and Z oblique → Monoclinic.
Step 4: Conclusion.
Hence, the correct pair is (B) Orthorhombic – Monoclinic.