Monochromatic Light vs Coherent Light
Monochromatic light and coherent light are two topics discussed under the modern day theory of light. These ideas play a major role in fields such as LASER technology, spectrophotometry and spectrometry, acoustics, neuroscience and even quantum mechanics. In this article, we are going to discuss what coherent and monochromatic light are, their definitions, the similarities and the differences between coherent light and monochromatic light.
The term “mono” refers to a singular object or subject. The term “chrome” refers to colors. The term “monochrome” is a reference to a single color. To understand monochromatic, one must first understand electromagnetic spectrum. Electromagnetic waves are classified into several regions according to their energy. X-rays, ultraviolet, infrared, visible, radio waves are to name a few of them. Everything we see is seen due to the visible region of the electromagnetic spectrum. A spectrum is the plot of intensity versus energy of the electromagnetic rays. The energy can also be represented in wavelength or frequency. A continuous spectrum is a spectrum in which all the wavelengths of the selected region have intensities. The perfect white light is a continuous spectrum over the visible region. It must be noted that, in practice, it is virtually impossible to obtain a perfect continuous spectrum. An absorption spectrum is the spectrum obtained after sending a continuous spectrum through some material. An emission spectrum is the spectrum obtained after the continuous spectrum is removed after the excitation of the electrons in the absorption spectrum.
Absorption spectrum and emission spectrum are very useful in finding chemical compositions of materials. The absorption or emission spectrum of a substance is unique to the substance. Since the quantum theory suggests the energy must be quantized, the frequency of the photon determines the energy of the photon. Since energy is discrete, the frequency is not a continuous variable. Frequency actually is a discrete variable. The color of a photon incident on the eye is determined by the energy of the photon. A ray having only photons of a single frequency is known as a monochromatic ray. Such a ray carries a beam of photons, which are same in color thus getting the term “monochromatic”.
Coherence is a property of light that enables waves to form temporary or stationary interference patterns. Coherence is defined to two waves. If two waves are monochromatic (having the same wavelength) and are of the same phase, these two waves are defined as coherent waves. Sources generating such waves are known as coherent sources. Such waves can be utilized to study the characteristics of the optical path. This is done by sending one ray through the desired path and sending the other as a control test.
What is the difference between Coherent light and Monochromatic light?
• Coherent light must have the same phase as well as the same frequency. Monochromatic light only has to have the same frequency.
• A coherent source is always monochromatic while a monochromatic source may or may not be a coherent source.
• Two separate sources can be practically used as monochromatic sources, but for coherence, two virtual sources designed from a single monochromatic source must be used.