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Infrared (IR) radiation is a type of electromagnetic energy that is prevalent in our daily lives, though it is invisible to the naked eye. It occupies a part of the electromagnetic spectrum that lies beyond the visible red light—hence 'infrared', which means 'below red'—with wavelengths ranging from about 700 nanometers (nm) to 1 millimeter (mm). This places IR radiation right between the visible spectrum and microwave radiation.
Electromagnetic spectrum refers to the entire range of electromagnetic radiation, which spans from short gamma rays to long radio waves. The part of the spectrum where a particular radiation lies is determined by its wavelength. In practical terms, IR radiation is commonly associated with heat, as objects emit this form of radiation dependent on their temperature. For instance, humans emit IR radiation, which is why we can be detected by thermal imaging cameras. IR radiation can be further subdivided into near-infrared (close to visible light), mid-infrared, and far-infrared (closer to microwaves). Applications of IR radiation include remote controls, night-vision equipment, and thermal insulation.

The energy of a photon can be thought of as a packet of electromagnetic radiation energy, and it is proportional to the photon's frequency. Since energy and frequency are directly related, with a decrease in wavelength, there comes an increase in both the frequency and the energy of the photons.
To calculate the energy of a photon, we use a basic formula derived from quantum physics:

\(E = h \frac{c}{\lambda}\)

where \(E\) is the energy of the photon in joules (J), \(h\) is Planck's constant (approximately \(6.626 \times 10^{-34} \mathrm{Js}\)), \(c\) represents the speed of light in a vacuum (which is about \(2.998 \times 10^8 \mathrm{m/s}\)), and \(\lambda\) is the wavelength of the photon in meters (m). This relationship is key for understanding how energy is quantized on a microscopic scale, such as in light-emitting diodes (LEDs) and lasers, which are vital in various technologies including communication devices and medical instruments. Knowing how many joules of energy are in a single photon allows us to calculate things like the number of photons emitted per second by a light source, given its total energy output.

Planck's constant is a fundamental quantity in quantum mechanics, symbolized by \(h\), and its value is approximately \(6.626 \times 10^{-34} \) joule-seconds (Js). This constant is remarkably important because it sets the scale of quantum effects, and connects the amount of energy a photon carries with the frequency of its electromagnetic wave.
Max Planck, a German physicist, introduced this constant in 1900 when explaining blackbody radiation, eventually leading to the birth of quantum mechanics. The constant has numerous applications in the field of physics, including the photoelectric effect, which provided evidence for the quantum nature of light and earned Albert Einstein the Nobel Prize in Physics in 1921. Planck's constant has such a fundamental role in quantum mechanics that the International System of Units (SI) has redefined the kilogram in terms of it, indicative of its well-established and precise value in physics. Understanding Planck's constant allows us to comprehend the discrete nature of energy levels in atoms and molecules, contrasting classical physics' interpretation of energy as a continuous spectrum.