Light travels as electromagnetic radiation, encompassing a vast spectrum of frequencies and wavelengths. Much of this energy remains invisible to the human eye. This article will define and compare two important segments of the invisible spectrum: Ultraviolet (UV) light and Infrared (IR) light. These two forms of radiation are fundamentally different in their physical properties and their interaction with the world.
Context of the Electromagnetic Spectrum
Electromagnetic radiation is energy propagated through space in the form of oscillating electric and magnetic fields. The electromagnetic spectrum is continuous, with each type of radiation distinguished by its frequency and wavelength. Frequency, measured in Hertz (Hz), represents the number of wave cycles passing a point per second. Wavelength is the physical distance between successive peaks of the wave.
These two properties are inversely related; a higher frequency corresponds to a shorter wavelength, and vice versa. The frequency of the radiation is directly proportional to the energy carried by its photons. Visible light occupies a small central section of this spectrum, with UV light on the higher-frequency side and IR light on the lower-frequency side.
Defining Ultraviolet Light
Ultraviolet (UV) light is the segment of the electromagnetic spectrum immediately beyond the violet end of visible light. The total UV range is defined by wavelengths from approximately 10 nanometers (nm) up to 400 nm. This corresponds to a high-frequency range, typically spanning from about \(8 \times 10^{14}\) Hz (800 Terahertz) up to \(3 \times 10^{16}\) Hz (30 Petahertz).
The UV spectrum is classified into three principal sub-bands based on wavelength. UVA (320 nm to 400 nm) is the closest to visible light, and nearly all UV radiation reaching the Earth’s surface is in this band.
UVB (280 nm to 320 nm) carries more energy than UVA and is responsible for sunburn and biological effects on the skin. UVC (100 nm to 280 nm) has the highest energy. UVC is almost entirely absorbed by the atmosphere’s ozone layer, preventing it from reaching the Earth’s surface.
Defining Infrared Light
Infrared (IR) light occupies the electromagnetic spectrum with wavelengths longer than visible red light but shorter than microwaves. The total IR range is defined by wavelengths from about 700 nanometers (nm) to 1 millimeter (mm). This corresponds to a frequency range generally extending from approximately 300 Gigahertz (GHz) up to 400 Terahertz (THz). IR radiation is often perceived as heat because objects near room temperature emit energy primarily in this band.
The IR spectrum is commonly subdivided into three regions. Near-Infrared (NIR) covers 700 nm to about 2,500 nm (2.5 \(\mu\)m), with frequencies from 120 THz to 430 THz. NIR is often used in fiber-optic communications.
Mid-Infrared (MIR) spans 2.5 \(\mu\)m to 12 \(\mu\)m (25 THz and 120 THz). This region is important for gas sensing and molecular spectroscopy. Far-Infrared (FIR) covers the longest IR wavelengths, from 12 \(\mu\)m up to 1 mm (3 THz to 25 THz), and is associated with thermal imaging and the heat emitted by living bodies.
Comparing Energy Levels and Practical Applications
The difference in frequency between UV and IR light results in a fundamental contrast in their photon energy levels and how they interact with matter. UV light, with its much higher frequency, possesses greater photon energy, allowing it to induce chemical changes. This photochemical effect means UV can break chemical bonds, a property utilized in applications such as:
- Germicidal sterilization, where UVC light destroys the DNA of microbes.
- Causing many substances to fluoresce.
- UV curing for inks and adhesives.
Conversely, IR light has a lower frequency and lower photon energy, causing molecules primarily to vibrate and rotate. This interaction is experienced as heat, making IR radiation largely thermal in nature. Practical applications for IR light focus on this thermal property, including:
- Thermal imaging cameras to visualize temperature differences.
- Industrial heating processes like drying and curing materials.
- Remote controls and fiber-optic telecommunications (using the NIR band).