What are the different uses of lasers?
The development of modern lasers has opened many doors to both research and applications. A laser beam was used to measure the distance from the Earth to the moon. Lasers are important components of CD players. As the image above illustrates, lasers can provide precise focusing of beams to selectively destroy cancer cells in patients. The ability of a laser to focus precisely is due to highquality crystals that help give rise to the laser beam. A variety of techniques are used to manufacture pure crystals for use in lasers.
Crystalline Solids
The majority of solids are crystalline in nature. A crystal is a substance in which the particles are arranged in an orderly, repeating, threedimensional pattern. Particles of a solid crystal may be ions, atoms, or molecules, depending on the type of substance. The threedimensional arrangement of a solid crystal is referred to as the crystal lattice. Different arrangements of the particles within a crystal cause them to adopt several different shapes.
Crystal Systems
Crystals are classified into general categories based on their shapes. A crystal is defined by its faces, which intersect with one another at specific angles, which are characteristic of the given substance. The seven crystal systems are shown below, along with an example of each. The edge lengths of a crystal are represented by the letters \begin{align*}a\end{align*}, \begin{align*}b\end{align*}, and \begin{align*}c\end{align*}. The angles at which the faces intersect are represented by the Greek letters \begin{align*}\alpha\end{align*}, \begin{align*}\beta\end{align*}, and \begin{align*}\gamma \end{align*}. Each of the seven crystal systems differs in terms of the angles between the faces and in the number of edges of equal length on each face.
Crystal System  Diagram  Example 
Cubic \begin{align*}a = b = c; \; \alpha = \beta = \gamma = 90^\circ\end{align*} 


Tetragonal \begin{align*}a = b \ne c; \; \alpha = \beta = \gamma = 90^\circ\end{align*} 


Orthorhombic \begin{align*}a \ne b \ne c; \; \alpha = \beta = \gamma = 90^\circ\end{align*} 
Aragonite 

Monoclinic \begin{align*}a \ne b \ne c; \; \alpha \ne 90^\circ = \beta = \gamma\end{align*} 
Azurite 

Rhombohedral \begin{align*}a = b = c; \; \alpha = \beta = \gamma \ne 90^\circ\end{align*} 
Calcite 

Triclinic \begin{align*}a \ne b \ne c; \; \alpha \ne \beta \ne \gamma \ne 90^\circ\end{align*} 
Microcline 

Hexagonal \begin{align*}a = b \ne c; \; \alpha = \beta = 90^\circ, \; \gamma = 120^\circ\end{align*} 

Summary
 A crystal is a substance in which the particles are arranged in an orderly, repeating, threedimensional pattern.
 The crystal lattice is the threedimensional arrangement of a solid crystal.
Review
 What is a crystal?
 List the seven crystal systems.