Two waves that are 180° out of phase will have one wave peaking at the same point in space that the other wave is bottoming out. The phase of a wave describes what part of a sine wave's cycle exists at a certain reference point. The waves have to be in phase (spectral coherence) Using monochromatic light can allow us to measure or trigger a very specific response in a material (e.g. In fact, having a light beam that is very close to single-wavelength (called "monochromatic") is one of the main reason lasers are so useful. Despite the fact that an exactly single-wavelength beam of light is physically impossible, we can get very, very close. It would take an infinitely long beam of light in order to have all the wave components have exactly the same wavelength (the proof of this statement is not obvious and requires Fourier analysis). Such a situation is actually physically impossible. Ideally, if all the wave components had exactly the same wavelength (and the other criteria listed below were met), then all the peaks could be lined up perfectly, forever. If one wave has its consecutive peaks separated by a distance of 600 nanometers, and another wave has its peaks separated by a distance of 830 nanometers, then it should be obvious that if you line up one pair of peaks, you cannot line up any of the other pairs of peaks. The waves have to have approximately the same wavelength (temporal coherence) In order for the peaks to stay completely aligned everywhere, a few things have to happen:ġ.
By the phrase "lined up", we mean that if you were able to take a snapshot at a certain time of the different wave components in a light beam, you would find that all the first peaks are at the same location in space, all the second peaks are at the same distance, etc. In this picture, coherence means that all the respective peaks of the various sine waves are lined up in space, and continue to stay lined up as the waves travel. What is coherence? In the simplest picture, you can visualize a beam of light as a bundle of many little sine waves traveling through space. In fact, the word "laser" is actually an acronym that stands for "Light Amplification by Stimulated Emission of Radiation" 1. Stimulated emission is what causes the light in a laser beam to be coherent, and coherence is what makes a laser beam so much more useful than regular light. You create a laser beam using stimulated emission. Furthermore, you can't create a laser beam by cleverly focusing regular light, no matter how hard you try. Baird.Ī laser beam is not just focused light. Public Domain Image, source: Christopher S. A laser beam is coherent light, not focused light.