Caesium n. : A rare alkaline metal found in mineral water; -- so called from the two characteristic blue lines in its spectrum. It was the first element discovered by spectrum analysis, and is the most strongly basic and electro-positive substance known. Symbol Cs. Atomic weight 132.6.

Example:
light from a laser is directed at a Caesium surface?

Taxonomy
Caesium, alkali metals, metal, isotopes, Melting Point, Boiling Point, Electronegativity, Atomic Number, Atomic Weight, mass, cm3, element, compounds, stable isotopes, caesium chloride, Isotopes of caesium, Caesium hydroxide, kJ.

Defination from the web:
Caesium is also used in atomic clocks, which are accurate to seconds over many thousands of years. Since 1967, the International System of Measurements has based its unit of time, the second, on the properties of caesium. ...