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Electromagnetic Spectrum

Electromagnetic Spectrum

Electromagnetic waves exist over an enormous range of frequencies and wavelengths. The classification of electromagnetic waves according to their frequency or wavelength is known as the electromagnetic spectrum. All forms of electromagnetic radiation travel at the speed of light ($c$) in vacuum, which is approximately $3 \times 10^8 \, m/s$. The frequency ($f$ or $\nu$) and wavelength ($\lambda$) of an electromagnetic wave are related by the fundamental equation:

$ c = f \lambda $

As frequency increases, wavelength decreases proportionally, and vice versa. The electromagnetic spectrum is continuous, but it is conventionally divided into different regions or bands based on how the waves are produced and detected, and their common applications. Starting from low frequency (long wavelength) to high frequency (short wavelength), the main regions of the electromagnetic spectrum are:

  1. Radio Waves
  2. Microwaves
  3. Infrared Radiation
  4. Visible Light
  5. Ultraviolet Radiation
  6. X-rays
  7. Gamma Rays

Diagram of the Electromagnetic Spectrum showing different regions by frequency and wavelength

The Electromagnetic Spectrum, showing different regions and their approximate frequency/wavelength ranges.

Radio Waves

Frequency Range: Generally, from a few Hz to about $10^9$ Hz (1 GHz). This range includes very low frequency (VLF), low frequency (LF), medium frequency (MF), high frequency (HF), very high frequency (VHF), ultra-high frequency (UHF), super high frequency (SHF), and extremely high frequency (EHF).

Wavelength Range: From very long wavelengths (hundreds or thousands of kilometres) down to about 0.3 meters.

Sources: Produced by the accelerated motion of charges in conducting wires or circuits. Oscillating electric circuits (like LC oscillators) and antennas are primary sources.

Properties: Radio waves are easily diffracted around obstacles and reflected by the ionosphere (for some frequency bands), allowing for long-distance communication. They can penetrate non-conducting materials.

Uses:

Microwaves

Frequency Range: From about 1 GHz to 300 GHz.

Wavelength Range: From about 0.3 meters down to about 1 millimetre.

Sources: Produced by specific electronic devices like klystrons, magnetrons, and Gunn diodes.

Properties: Microwaves can penetrate certain materials but are absorbed by water molecules, leading to heating. They travel in straight lines and are easily reflected by metallic surfaces.

Uses:

Infrared Waves

Frequency Range: From about 300 GHz to $4 \times 10^{14}$ Hz.

Wavelength Range: From about 1 millimetre down to about 750 nanometres (nm). This range is sometimes divided into near, mid, and far infrared.

Sources: Emitted by hot objects (molecules and atoms in vibration). The human body emits infrared radiation.

Properties: Infrared radiation is often felt as heat. It is absorbed by molecules, increasing their vibrational energy. It can pass through fog and smoke better than visible light.

Uses:

Visible Rays (Visible Light)

Frequency Range: From about $4 \times 10^{14}$ Hz to $7.5 \times 10^{14}$ Hz.

Wavelength Range: From about 750 nm (red) down to about 400 nm (violet).

Sources: Produced by the excitation and de-excitation of electrons in atoms and molecules (e.g., in light bulbs, stars, LEDs).

Properties: This is the portion of the spectrum that is detectable by the human eye. Different frequencies within this range are perceived as different colours (VIBGYOR - Violet, Indigo, Blue, Green, Yellow, Orange, Red). It exhibits phenomena like reflection, refraction, interference, diffraction, and polarisation.

Uses:

Ultraviolet Rays (UV)

Frequency Range: From about $7.5 \times 10^{14}$ Hz to $3 \times 10^{16}$ Hz.

Wavelength Range: From about 400 nm down to about 10 nm.

Sources: Produced by excited atoms and molecules, electric arcs, special lamps, and the Sun. The Sun is a significant source of UV radiation.

Properties: UV radiation is more energetic than visible light. It can cause chemical reactions, including ionisation. It is largely absorbed by the Earth's ozone layer. Excessive exposure can be harmful to living organisms (sunburn, skin damage).

Uses:

X-Rays

Frequency Range: From about $3 \times 10^{16}$ Hz to $3 \times 10^{19}$ Hz.

Wavelength Range: From about 10 nm down to about 10 picometres (pm) ($10^{-11}$ m).

Sources: Produced when high-speed electrons are suddenly decelerated upon striking a metal target (bremsstrahlung) or by electron transitions within the inner shells of heavy atoms.

Properties: X-rays are very energetic and highly penetrating. They can pass through soft tissues of the body but are absorbed by denser materials like bone and metal. They cause ionisation. Prolonged exposure is harmful.

Uses:

Gamma Rays

Frequency Range: Above $3 \times 10^{19}$ Hz.

Wavelength Range: Below about 10 pm.

Sources: Produced during radioactive decay of atomic nuclei and in high-energy nuclear reactions (like those occurring in stars or supernovae).

Properties: Gamma rays are the most energetic form of electromagnetic radiation and are extremely penetrating. They are highly ionising and very harmful to living tissues.

Uses:

The different regions of the electromagnetic spectrum are not sharply defined and often overlap. The boundaries are nominal and based on typical sources and detection methods rather than fundamental differences in the nature of the radiation. Understanding the electromagnetic spectrum is key to appreciating the diverse ways in which energy is carried through space and interacts with matter.