Ep 01 — What is Infrared Light? From Visible to the Invisible World
Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Chapter: Part 1 · Introduction — The Code of Light
Audience: High school students, undergraduates, beginners in chemistry/materials/pharmaceutical science
Prerequisites: None
Reading time: ~15 minutes
Introduction: A Thermometer Reveals the Hidden World
In the spring of 1800, the British astronomer William Herschel (1738–1822) was working on a problem seemingly unrelated to astronomy—he wanted to find a suitable glass filter that would allow him to safely observe the Sun without burning his eyes. He noticed that different colored filters transmitted varying amounts of heat. So he designed an experiment: he used a prism to disperse sunlight into a spectrum, then placed blackened thermometers in different color regions of the spectrum to measure temperature [1].
Herschel used three mercury thermometers (with their bulbs blackened to better absorb heat): one in the violet light region, one in the red light region, and a third placed just beyond the red—in a region completely dark to the naked eye, as a control [2].
The result astonished him: The thermometer in that "dark" region recorded a higher temperature than any of the colored regions of the visible spectrum [1][2].
"I placed a thermometer in the region beyond the red light... and found that the temperature in that region was higher than in any part of the visible spectrum."
— William Herschel, paper read to the Royal Society on March 27, 1800 [3]
Herschel called this invisible radiation "calorific rays" (from Latin calor, meaning "heat"). He further demonstrated that these rays, like visible light, could be reflected, refracted, absorbed, and transmitted [1]. The term "infrared" only entered scientific vocabulary in the 1880s, where infra is Latin for "below," referring to its frequency being lower (i.e., wavelength longer) than red light [3].
The historical significance of Herschel's experiment lies not only in the discovery of infrared radiation—after all, people had long known that one could feel warmth from a distant heat source—but in providing the first experimental evidence that "light and heat are the same physical entity" [3]. This discovery laid the groundwork for James Clerk Maxwell's later unification of electromagnetic theory.
📷 Figure 1: Herschel's experiment diagram — a prism disperses sunlight into a spectrum, with thermometers placed in different color regions and beyond the red
Source: NASA/IPAC Caltech education page [2]
https://www.ipac.caltech.edu/…
I. The Electromagnetic Spectrum: Position of Infrared Light
To understand infrared light, we need to place it in the broader picture—the Electromagnetic Spectrum.
The electromagnetic spectrum encompasses all electromagnetic radiation arranged by wavelength (or frequency). From gamma rays with the shortest wavelengths and highest energy to radio waves with the longest wavelengths and lowest energy, the spectrum spans at least 16 orders of magnitude [4].
| Band | Wavelength Range | Frequency Range | Main Interaction |
|---|---|---|---|
| Gamma rays | <0.01 nm | >30 EHz | Nuclear transitions |
| X-rays | 0.01–10 nm | 30 PHz–30 EHz | Inner shell electron ionization |
| Ultraviolet | 10–380 nm | 790 THz–30 PHz | Outer shell electron excitation |
| Visible | 380–780 nm | 385–790 THz | Visual perception |
| Infrared | 780 nm–1 mm | 300 GHz–385 THz | Molecular vibrations |
| Microwave | 1 mm–1 m | 300 MHz–300 GHz | Molecular rotations |
| Radio waves | >1 m | <300 MHz | Nuclear spins (NMR) |
Table 1: Regions of the electromagnetic spectrum and their interactions with matter (Data sources: Bruker FTIR Basic Guide [5]; KCL Infrared Spectroscopy Lecture Notes [6])
📷 Figure 2: Full spectrum of electromagnetic waves
Source: Wikimedia Commons (public domain)
https://upload.wikimedia.org/…
Key insight: The visible light that our eyes can detect constitutes only a tiny fraction of the entire electromagnetic spectrum. The wavelength range of infrared light is from 780 nm to 1 mm—over 1000 times the width of the visible band [5].
II. Three Subdivisions of Infrared: Near-, Mid-, and Far-Infrared
Infrared light is not a uniform whole. Based on wavelength and the mechanisms of interaction with matter, it is further divided into three subregions [5][6][7]:
| Region | Abbreviation | Wavelength Range | Wavenumber Range | Key Feature | Typical Applications |
|---|---|---|---|---|---|
| Near-infrared | NIR | 780 nm – 2.5 μm | 12,800–4,000 cm⁻¹ | High penetration | Agricultural quality, brain functional imaging |
| Mid-infrared | MIR | 2.5 – 25 μm | 4,000–400 cm⁻¹ | High specificity | Molecular structure identification (infrared spectroscopy) |
| Far-infrared | FIR | 25 μm – 1 mm | 400–10 cm⁻¹ | Strong thermal effects | Inorganic materials, lattice vibrations |
Table 2: Three subregions of infrared light (Data sources: CSDN Infrared Family Overview [7]; KCL Lecture Notes [6]; Columbia University Lab Manual [8])
2.1 Why is the Mid-Infrared the "Chemist's Main Battlefield"?
The mid-infrared (4,000–400 cm⁻¹) occupies a central role in chemical analysis because the vibrational frequencies of chemical bonds in molecules happen to fall within this range [5][6].
When infrared light matches the vibrational frequency of a chemical bond in a molecule, the molecule absorbs that energy and transitions from the ground state to an excited state—this is fundamental absorption [6]. Since different functional groups (e.g., C=O, O-H, C-H) have different vibrational frequencies, they produce characteristic absorption peaks at distinct positions in the mid-infrared region. This gives each molecule a unique "fingerprint" in the mid-infrared, making mid-infrared spectroscopy one of the most powerful tools for identifying chemical structures [5].
A popular science article on CSDN used a vivid analogy [7]:
"Mid-infrared is like a molecule's 'ID card'—a sharp strong peak around 1700 cm⁻¹ strongly suggests the presence of a carbonyl group (C=O), but must be cross-validated with peak shape and the fingerprint region."
📷 Figure 3: Comparison of physical mechanisms for near-, mid-, and far-infrared
Source: CSDN Deep Reading Series [7] (used with permission)
https://blog.csdn.net/mengjiz…
2.2 What about Near- and Far-Infrared?
Near-infrared (NIR) primarily corresponds to overtones and combination bands of molecular vibrations. The signals are weak (typically about 1% of fundamental absorption), but due to the weak absorption, light penetrates deeply, making it suitable for non-destructive, deep detection [7]. For example:
- Supermarket NIR sensors measure sugar content in fruit
- Functional near-infrared spectroscopy (fNIRS) monitors blood oxygen changes in the cerebral cortex through the skull [7]
Far-infrared (FIR) corresponds to molecular rotations and lattice vibrations (phonons), with lower energy. It is commonly used in studies of inorganic materials, crystalline structures, and organometallic compounds [6][8].
III. Wavenumber (cm⁻¹): The "Common Language" of Spectroscopists
If you look at any infrared spectrum, you'll notice the horizontal axis is not labeled with wavelength (nm or μm) but with a less common unit—wavenumber, in units of cm⁻¹ (how many waves per centimeter).
3.1 What is Wavenumber?
Wavenumber (denoted ν̃) is the reciprocal of wavelength, defined as the number of waves per unit length [9]:
$$\tilde{\nu} = \frac{1}{\lambda}$$
When wavelength is in μm, the conversion formula is:
$$\tilde{\nu} \text{(cm}^{-1}\text{)} = \frac{10{,}000}{\lambda \text{(μm)}}$$
For example:
- The characteristic absorption of carbonyl (C=O) is around 1700 cm⁻¹ → wavelength = 10,000 / 1700 ≈ 5.88 μm
- Water (O-H) absorption is around 3400 cm⁻¹ → wavelength = 10,000 / 3400 ≈ 2.94 μm
- Visible green light at 532 nm → wavenumber = 10,000,000 / 532 ≈ 18,797 cm⁻¹ [10]
3.2 Why use wavenumber instead of wavelength?
Spectroscopists prefer wavenumber over wavelength for four core reasons [9][10][11]:
1. Wavenumber is proportional to energy – more intuitive
Photon energy E = hcν̃, where h is Planck's constant and c is the speed of light. Since h and c are constants, energy is directly proportional to wavenumber: larger wavenumber means higher energy [10]. Wavelength, on the other hand, is inversely proportional to energy (E = hc/λ), which is less intuitive.
2. Energy differences can be directly added or subtracted
In molecular spectroscopy, energy differences expressed in cm⁻¹ can be calculated by simple addition or subtraction: Δν̃ = ν̃₂ − ν̃₁. Using wavelength requires taking reciprocals first, which is cumbersome and unintuitive [9].
3. Uniform distribution on the spectral axis
When spectra are plotted with wavenumber on the horizontal axis, the spacing between peaks is proportional to the energy spacing, making the plot visually uniform and reasonable. If wavelength is used, the short-wavelength region is compressed and the long-wavelength region stretched, which is inconvenient for comparing spectra [9].
4. Historical convention and numerical convenience
The mid-infrared wavelength range is 2.5–25 μm, which converts conveniently to wavenumbers of 4,000–400 cm⁻¹ – a moderate numerical range, easy to record and communicate [10].
📷 Figure 4: Comparison of the same spectrum plotted with wavenumber and wavelength as the horizontal axis
Source: WavelengthCalculator Spectral Guide [10]
https://www.wavelengthcalcula…
3.3 An interesting historical question: Why are infrared spectra plotted from high to low?
If you look closely at an infrared spectrum, you'll notice the horizontal axis is arranged from high wavenumber (left) to low wavenumber (right) – opposite to other spectra (e.g., UV-Vis spectra are usually from low to high) [12].
This "counterintuitive" arrangement has historical roots. Early infrared spectrometers used prisms for dispersion, and spectra were recorded by scanning from short to long wavelength (i.e., high to low wavenumber). Around 1905, infrared spectroscopy pioneer William Coblentz plotted his spectra with wavelength increasing from left to right in his multi-volume work "Investigations of Infra-Red Spectra" [12].
Later, when the scientific community switched from wavelength to wavenumber as the unit for the horizontal axis, the convention of "high to low" was retained to maintain the continuity of spectral appearance (so that peaks originally on the left remained on the left) [12]. Both IUPAC and the Coblentz Society have followed this convention [12].
"People were already accustomed to looking at spectra with wavelength increasing from left to right. When they later switched to wavenumber, since wavenumber is inversely proportional to wavelength, it naturally became high to low."
– ResearchGate academic discussion [12]
IV. Infrared light in daily life
Infrared light is not only found in the laboratory – it is ubiquitous.
| Scenario | Infrared type | Principle |
|---|---|---|
| TV remote control | Near-infrared | IR LED emits 940 nm signal |
| Thermal imager | Mid-/Far-infrared | Detects thermal radiation from objects (8–14 μm) |
| Night vision device | Near-infrared | Enhances ambient infrared light |
| Oven/heater | Mid-/Far-infrared | Thermal radiation heating |
| Automatic sensor door | Near-infrared | Infrared reflection detection |
| Ear thermometer | Far-infrared | Detects thermal radiation from eardrum |
| Fiber optic communication | Near-infrared | Low-loss transmission at 1310/1550 nm |
Table 3: Infrared applications in daily life (Data sources: NASA/IPAC educational resources [2]; Bruker guide [5])
Any object with a temperature above absolute zero (−273.15 °C) emits infrared radiation – this is called thermal radiation. The peak wavelength of thermal radiation from the human body (about 37 °C) is around 9.7 μm, falling in the far-infrared region [2]. That is why thermal imagers can "see" people in complete darkness without an external light source – they detect the infrared radiation emitted by the body itself.
📷 Figure 5: Infrared thermography photo of a house (warmer colors indicate higher temperature areas)
Source: Wikimedia Commons (CC BY-SA 3.0)
https://upload.wikimedia.org/…
V. Why is mid-infrared (2.5–25 μm) the main battlefield for molecular analysis?
Back to our key question: Why do chemists almost exclusively use mid-infrared?
The answer lies in a critical physical correspondence:
The vibrational frequencies of chemical bonds in molecules happen to fall within the mid-infrared band.
Specifically [5][6][8]:
Chemical bonds are like springs: The chemical bond in a molecule can be compared to a spring connecting two atomic balls. The masses of the atoms (m) and the force constant of the bond (k, the "spring stiffness") determine the vibrational frequency.
Frequency matching: The energy of mid-infrared photons (about 4.8–48 kJ/mol, corresponding to 400–4000 cm⁻¹; roughly 1.1–11.5 kcal/mol) exactly matches the energy required for a molecule to transition from the ground vibrational level to the first excited level [8].
Resonant absorption: When the frequency of infrared light matches the molecular vibrational frequency, resonant absorption occurs – the molecule absorbs the photon and its vibration amplitude increases [6].
Frequency uniqueness: Different chemical bonds (C-H, C=O, O-H, N-H, etc.) have different force constants and atomic masses, so their vibrational frequencies differ, appearing as absorption peaks at different positions in the mid-infrared spectrum [5].
This is the fundamental reason why infrared spectroscopy can identify molecular structures: each molecule's combination of vibrational modes is unique, like a human fingerprint. In subsequent lessons, we will delve into the physical models of molecular vibrations and the characteristic frequencies of various functional groups.
If you want to look up the characteristic infrared absorption frequencies of specific functional groups, you can visit the functional group page and frequency table at ftir.fun:
- 📊 Functional group frequency reference table: https://ftir.fun/ir/frequency…
- 🔍 Search by peak position (e.g., what functional group corresponds to 1700 cm⁻¹): https://ftir.fun/ir/peak/1700/
- 🏷️ Search by functional group (e.g., characteristic peak of carbonyl C=O): https://ftir.fun/ir/group/car…
Summary of this episode
| Core knowledge point | Key points |
|---|---|
| Discovery of infrared light | Discovered in 1800 by Herschel using prism + thermometer experiment |
| Position in the electromagnetic spectrum | Wavelength 780 nm–1 mm, between visible light and microwaves |
| Three infrared subregions | Near-IR (high penetration), Mid-IR (high characteristic absorption), Far-IR (thermal effects) |
| Central role of mid-IR | Vibrational frequencies of chemical bonds happen to fall in this region (4,000–400 cm⁻¹) |
| Wavenumber (cm⁻¹) | Reciprocal of wavelength, proportional to energy, standard unit in spectroscopy |
| Spectral plotting convention | From high wavenumber (left) to low wavenumber (right), historical habit |
Questions for thought
- In Herschel's experiment, why did the thermometer need to be blackened?
- If the wavelength of infrared light is 6.67 μm, what is the corresponding wavenumber? Does it belong to near-infrared or mid-infrared?
- Why are absorption peaks in near-infrared spectra usually weaker than those in mid-infrared?
- Can you think of other scenarios in daily life that utilize infrared light?
References
[1] NASA/IPAC Caltech. "Herschel Discovers Infrared Light." Cool Cosmos Educational Site.
https://www.ipac.caltech.edu/…
[2] NASA/IPAC Caltech. "The Herschel Experiment." Cool Cosmos Educational Site.
https://www.ipac.caltech.edu/…
[3] White, J. R. "Herschel and the Puzzle of Infrared." American Scientist, 2012, 100(3): 218. DOI: 10.1511/2012.96.218.
https://www.americanscientist…
[4] SAC-CSIC. "Herschel Experiment: The Discovery of the Infrared." International Day of Light Educational Resource, 2018.
http://sac.csic.es/astrosecun…
[5] Bruker Optics. "Guide to Infrared Spectroscopy." FTIR Basics.
https://www.bruker.com/produc…
[6] King's College London. "Infrared Spectroscopy." Pharmacy Teaching Handout.
https://tst.rcsv4webapp.kcl.a…
[7] mengjizhiyou. "In-depth Concept: The Infrared Family — The 'Romance of the Three Kingdoms' of Near-, Mid-, and Far-Infrared." CSDN Blog, 2026-02-04.
https://blog.csdn.net/mengjiz…
[8] Columbia University. "Experiment 7: IR Spectroscopy." Undergraduate Chemistry Lab Manual.
http://www.columbia.edu/cu/ch…
[9] kaixin_啊啊. "Wavelength and Wavenumber in Spectral Data." CSDN Blog, 2025-11-30.
https://blog.csdn.net/m0_7387…
[10] WavelengthCalculator.org. "Wavenumber to Wavelength Conversion: Spectroscopy Guide."
https://www.wavelengthcalcula…
[11] LibreTexts. "Infrared Spectroscopy." Chemistry LibreTexts, Section 3.12.
https://chem.libretexts.org/@…
[12] Brown, L. V. "Does anyone know why FTIR spectra are typically plotted from high to low wavenumbers?" ResearchGate Discussion, 2017.
https://www.researchgate.net/…
Preview of Next Episode: Ep 02 — Why Do Molecules "Eat" Light? The Nature of Molecular Vibrations and Infrared Absorption
We will delve into the molecular interior, using a spring-ball model to understand chemical bond vibrations, explore quantized vibrational energy levels, and the physical process of resonance absorption.
This article is licensed under CC BY-NC-SA 4.0. Figures are from public domain or labeled online resources, copyrights belong to the original authors.