What is a Monochromatic Light?

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- GoPhotonics

Aug 14, 2023

Monochromatic light is defined as light consisting of only one wavelength. It is also known as single-wavelength light, which derived its name from "mono" meaning single, and "chroma" denoting color. Monochromatic lights primarily fall within the visible light spectrum, featuring a narrow band of wavelengths. These lights possess a wavelength confined to a short range, and their unique attributes include intensity or brightness, color, propagation direction, and polarization state.

Source of Monochromatic Light

The light from a LASER (Light Amplification by Stimulated Emission of Radiation) is a well-known example of monochromatic light, which means it has only one color. A single atomic transition gives rise to the emission of light by a laser, resulting in the creation of a distinct and specific wavelength, thus giving birth to a precise monochromatic light beam. The opposite of monochromatic is polychromatic. Polychromatic light is commonly seen in everyday life as thermal radiation, such as that emitted by an incandescent lamp. This type of light covers a broad range of optical frequencies, displaying a variety of colors and wavelengths.

A monochromator can distinguish monochromatic light from polychromatic light. Before the advent of lasers, generating monochromatic light was a challenging task. One approach involved using certain gas-discharge lamps and metal vapor lamps (like mercury vapor lamps and sodium vapor lamps) that emitted light primarily in specific narrow spectral lines. These lamps allowed for isolating one of these lines using a suitable monochromator. However, the optical powers and intensities achieved were generally quite low.

Generation of Monochromatic Light

Monochromatic light can be generated through various methods, with the most common and effective approach being the use of a LASER (Light Amplification by Stimulated Emission of Radiation). Lasers produce highly coherent and single-wavelength light, making them a prominent source of monochromatic light. Another method involves the use of interference filters, which selectively transmit light of a specific wavelength while blocking others. Spectroscopy techniques can also be employed to disperse white light into its component colors and then select a single wavelength from the spectrum. Also, some LASERs can be tuned to emit light at different wavelengths, offering flexibility in generating monochromatic light. 

Nonlinear optical processes provide another way to create new wavelengths by interacting with a fundamental monochromatic light source. The generation of monochromatic light is essential in various applications, including scientific research, medical diagnostics, telecommunications, and optical imaging. The choice of method depends on the specific wavelength requirements and the intended use of the monochromatic light.

Properties of Monochromatic Light

  • Narrow bandwidth of wavelengths
  • High intensity 
  • Travel vast distances in a straight line
  • Appears as a single color in the visible region
  • High amount of energy
  • Focused within a very narrow area and cannot be dispersed from the point of focus

Applications of Monochromatic Light

Monochromatic light finds a wide range of applications across various scientific, industrial, and technological fields. One important application is in spectroscopy, where monochromatic light is used to study the interaction of matter with specific wavelengths. This technique enables the identification and analysis of chemical compounds, the determination of molecular structures, and the investigation of physical properties such as absorption, emission, and scattering. In medicine, monochromatic light is employed in phototherapy for treating conditions like jaundice in newborns and certain skin disorders, leveraging the therapeutic effects of specific wavelengths to stimulate biological processes and promote healing.

It also plays a crucial role in optical communication and data transmission. In fiber-optic communication systems, lasers produce monochromatic light signals that carry digital information over long distances through optical fibers. The use of a single wavelength minimizes signal interference and allows for high data transmission rates and low signal loss. Monochromatic light is also unavoidable in precision metrology, helping in the measurement of distances, angles, and other physical quantities with utmost accuracy. This is particularly valuable in fields like surveying, astronomy, and nanotechnology, where precise measurements are essential for scientific advancements and technological innovations.