The Light we see is just one type of light on a large spectrum 

The electromagnetic spectrum spans a vast range, from the low frequencies used in modern radio communications to the high-frequency gamma radiation at the short-wavelength end. It encompasses wavelengths ranging from thousands of kilometres down to a fraction of an atom's size. The longest wavelength limit corresponds to the size of the universe itself, while the shortest wavelength is believed to approach the Planck length, although in theory, the spectrum is infinite and continuous.

Light as a source of food

Autotrophs

Autotrophs are organisms capable of producing complex organic compounds like carbohydrates, fats, and proteins from simple inorganic molecules, utilizing energy from either light or chemical reactions. They are the primary producers in ecosystems, such as plants on land and algae in aquatic environments. Autotrophs synthesize their own food, using carbon fixation to convert carbon dioxide into organic compounds, thus eliminating the need for external organic sources of carbon or energy. Most autotrophs use water as a reducing agent in this process, though some can use other compounds like hydrogen sulfide.

Autotrophs are classified into two main groups:

• Phototrophs: These organisms harness energy from light to fuel their metabolic processes.

• Lithotrophs: These organisms derive energy by oxidizing inorganic substances, such as hydrogen sulfide, sulfur, ammonium, or ferrous iron. Both phototrophs and lithotrophs use energy (in the form of ATP) to reduce carbon dioxide and form organic molecules, such as NADPH, during photosynthesis or chemical oxidation.

Heterotrophs

Heterotrophs are organisms that cannot produce their own food and must obtain organic carbon by consuming other organisms—either autotrophs or other heterotrophs. They break down the complex organic compounds created by autotrophs, thus playing the role of consumers in the food chain.

Animals, fungi, and many bacteria are heterotrophs. Some organisms, such as corals, form symbiotic relationships with autotrophs to acquire organic carbon. Additionally, some parasitic plants have adopted partial or complete heterotrophy, while carnivorous plants supplement their nitrogen intake by consuming animals, although they remain autotrophic.

Heterotrophs are divided into two main types:

• Photoheterotrophs: These organisms use light as an energy source but require organic compounds for carbon. Many bacteria fall into this category.

• Chemoheterotrophs: These organisms rely entirely on organic compounds for both their energy and carbon needs. All animals, fungi, and many bacteria are chemoheterotrophs.

The eletromagentic spectrum

Electromagnetic waves are typically characterised by three physical properties: frequency (f), wavelength (λ), or photon energy (E). Frequencies extend from around 2.4×10²³ Hz (such as 1 GeV gamma rays) to the local plasma frequency of the ionised interstellar medium, approximately 1 kHz. Wavelength is inversely proportional to frequency, meaning that gamma rays, with their very high frequencies, have extremely short wavelengths—smaller than atoms—while radio waves can have wavelengths as vast as the universe. Photon energy is directly proportional to frequency, so gamma rays possess the highest energies (around a billion electron volts), while radio waves have very low energy. These relationships are captured by the following equations:

Where:

• c = 299,792,458 m/s is the speed of light in vacuum and

• h = 6.62606896(33)×10−34 J s = 4.13566733(10)×10−15 eV s is Planck's constant.

When electromagnetic waves travel through a medium containing matter, their wavelength decreases. The wavelengths of electromagnetic radiation are generally quoted based on their values in a vacuum, even when the waves are moving through different media, though this is not always explicitly stated.

Electromagnetic radiation is typically classified by its wavelength into categories such as radio waves, microwaves, infrared, the visible light spectrum (which we perceive as light), ultraviolet, X-rays, and gamma rays. The behaviour of EM radiation varies according to its wavelength, and when interacting with individual atoms or molecules, it also depends on the energy carried by each quantum (photon).

Spectroscopy, a technique used to analyse EM radiation, can detect a broader range of the electromagnetic spectrum than the visible light spectrum, which spans wavelengths from 400 nm to 700 nm. A standard laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm, providing detailed information about the physical properties of objects, gases, or stars. Spectroscopes are widely employed in astrophysics; for example, hydrogen atoms often emit a radio wave photon with a wavelength of 21.12 cm. Additionally, frequencies as low as 30 Hz, which are important for studying certain stellar nebulae, and as high as 2.9×10²⁷ Hz, have been detected from astrophysical sources.

Radio 

Radio waves, with wavelengths ranging from hundreds of meters to about one millimeter, are commonly utilized by antennas of corresponding sizes, based on the principle of resonance. These waves are widely used for data transmission via modulation in technologies like television, mobile phones, wireless networks, and amateur radio. The use of the radio spectrum is strictly regulated by governments through frequency allocation.

Information is carried by radio waves through variations in amplitude, frequency, and phase within a frequency band. When electromagnetic (EM) radiation interacts with a conductor, it induces an electric current by exciting electrons within the material, a principle utilized in antennas. EM radiation can also cause molecules to absorb energy, generating heat, which is the mechanism behind microwave ovens.

Microwaves 

Microwaves occupy the super high frequency (SHF) and extremely high frequency (EHF) regions of the spectrum. They are short enough to use tubular metal waveguides for transmission. Microwave energy is generated by devices like klystron and magnetron tubes, as well as solid-state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules with a dipole moment, particularly in liquids, which is how food is heated in microwave ovens.

Microwave heating works through volumetric heating, transferring energy electromagnetically rather than through thermal conduction. This method results in more uniform heating and drastically reduces cooking time, sometimes less than 1% of conventional methods. However, high-power microwave ovens can interfere with nearby electromagnetic fields, such as those found in poorly shielded medical devices or electronics.

Terahertz radiation

 Terahertz radiation lies between far infrared and microwaves. This frequency range was rarely studied until recently, but now it is being explored for applications in imaging and communications. Terahertz technology is also being investigated for military use, where high-frequency waves could be employed to disable enemy electronic equipment.

Infrared radiation 

Infrared radiation spans from approximately 300 GHz (1 mm) to 400 THz (750 nm) and is divided into three parts:

Far-infrared (300 GHz to 30 THz, or 1 mm to 10 μm): Absorbed by rotational modes in gas-phase molecules, molecular motions in liquids, and phonons in solids. Water vapor in Earth's atmosphere absorbs much of this radiation, making it opaque in this range, although some wavelength windows allow partial transmission, useful in astronomy.

Mid-infrared (30 to 120 THz, or 10 to 2.5 μm): Hot objects radiate strongly in this range, which is absorbed by molecular vibrations. This range is known as the "fingerprint region," as the absorption spectrum here is highly specific to individual compounds.

Near-infrared (120 to 400 THz, or 2.5 μm to 750 nm): Similar physical processes occur as with visible light.

Visible radiation  - the light that we see

Visible light comes just above infrared on the spectrum and is the range in which the Sun and similar stars emit most of their radiation. The human eye is attuned to these wavelengths, likely due to evolutionary adaptation. Visible light is absorbed and emitted by electrons transitioning between energy levels in atoms and molecules. This small portion of the EM spectrum is what humans perceive as light, with infrared and ultraviolet lying just beyond the red and violet ends of the visible spectrum, respectively.

When visible light reflects off objects, such as a bowl of fruit, it reaches our eyes, enabling us to see colors. Our visual system processes the reflected light frequencies into shades and hues. However, most EM radiation is beyond human perception, though technology allows us to manipulate a broad range of wavelengths, such as using optical fibers to transmit data.

Ultraviolet light 

Ultraviolet (UV) light has a wavelength shorter than visible light and longer than X-rays. UV radiation is energetic enough to break chemical bonds, making molecules unusually reactive or ionized. This effect is evident in sunburns, where UV damages DNA in skin cells, which can lead to skin cancer. The Sun emits large amounts of UV radiation, but most of it is absorbed by Earth's ozone layer.

X-rays 

X-rays come after UV on the spectrum and are highly ionizing. They interact with matter via the Compton effect and have a range of applications, from medical imaging to high-energy physics and astronomy. Hard X-rays have shorter wavelengths than soft X-rays and can pass through most materials, making them useful for diagnostic imaging. In astronomy, X-rays are emitted by neutron stars and black holes.

Gamma rays 

Gamma rays are the highest-energy photons, discovered by Paul Villard in 1900. They are useful in both astronomy and physics due to their penetrative power and their emission from radioisotopes. Gamma rays are used in food sterilization, radiation cancer therapy, and diagnostic imaging such as PET scans. Their wavelengths can be precisely measured using Compton scattering.

Spectrum Boundaries There are no strict boundaries between the different regions of the electromagnetic spectrum, and some types of radiation exhibit properties of multiple regions. For example, red light can resonate with certain chemical bonds, similar to infrared radiation.

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