Lasers are more than just powerful flashlights. The difference between ordinary light. And laser light is like the difference between ripples in your bathtub and huge waves on the sea. You’ve probably noticed that if you move your hands back and forth in the bathtub you can make quite strong waves. If you keep moving your hands in step with the waves you make, the waves get bigger and bigger. Imagine doing this a few million times in the open ocean. Before long, you’d have mountainous waves towering over your head! A laser does something similar with light waves. It starts off with weak light and keeps adding more and more energy so the light waves become ever more concentrated.

If you’ve even seen a laser in a science lab, you’ll have noticed two very important differences straightaway:

• Where a flashlight produces “white” light (a mixture of all different colors, made by light waves of all different frequencies). A laser makes what’s call monochromatic light (of a single, very precise frequency and color. Often bright red or green or an invisible “color” such as infrared or ultraviolet).
• Where a flashlight beam spreads out through a lens into a short and fairly fuzzy cone, a laser shoots a much tighter, narrower beam over a much longer distance (we say it’s highly collimated).

There’s a third important difference you won’t have noticed:

• Where the light waves in a flashlight beam are all jumbled up (with the crests of some beams mixed with the troughs of others), the waves in laser light are exactly in step: the crest of every wave is lined up with the crest of every other wave. We say laser light is coherent. Think of a flashlight beam as a crowd of commuters, pushing and shoving, jostling their way down the platform of a railroad station; by comparison, a laser beam is like a parade of soldiers all marching precisely in step.

These three things make lasers precise, powerful, and amazingly useful beams of energy.