5.2 The Electromagnetic Spectrum

The electromagnetic spectrum is a fascinating range of energy waves, from long radio waves to short gamma rays. Each type of radiation has unique properties and interactions with Earth's atmosphere, shaping our world and our ability to observe the cosmos.

Understanding the spectrum is crucial for grasping how we perceive light, heat, and energy in the universe. It explains why we see certain colors, how we detect distant objects, and even how our planet maintains its temperature through the greenhouse effect.

The Electromagnetic Spectrum

Bands of electromagnetic spectrum

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• Radio waves
  • Possess the longest wavelengths and lowest energy in the electromagnetic spectrum
  • Commonly used for transmitting radio and television signals over long distances
• Microwaves
  • Exhibit shorter wavelengths and higher energy compared to radio waves
  • Utilized in microwave ovens for heating food and in radar systems for detecting objects
• Infrared (IR)
  • Features shorter wavelengths and higher energy than microwaves
  • Emitted by objects with temperatures above absolute zero, used in night vision devices and remote sensing applications (weather satellites)
• Visible light
  • Comprises a small range of wavelengths that can be detected by the human eye
  • Colors span from red (longest visible wavelength) to violet (shortest visible wavelength) (ROYGBIV)
• Ultraviolet (UV)
  • Possesses shorter wavelengths and higher energy compared to visible light
  • Can cause damage to living tissues (sunburns) and is employed in sterilization processes (water purification)
• X-rays
  • Exhibit even shorter wavelengths and higher energy than UV radiation
  • Commonly used in medical imaging (radiographs) and security scanners (airports)
• Gamma rays
  • Represent the shortest wavelengths and highest energy in the electromagnetic spectrum
  • Generated by radioactive decay processes and high-energy cosmic events (supernovae, pulsars)

Electromagnetic spectrum and Earth's atmosphere

• Radio waves
  • Pass through Earth's atmosphere with minimal attenuation
  • Enable ground-based radio telescopes to observe celestial objects
• Microwaves
  • Largely pass through Earth's atmosphere unimpeded
  • Experience some absorption by atmospheric water vapor and oxygen molecules
• Infrared
  • Partially absorbed by Earth's atmosphere, primarily by water vapor and carbon dioxide
  • Contributes to the greenhouse effect, where atmospheric gases absorb and re-emit IR radiation, warming the planet
• Visible light
  • Mostly transmits through Earth's atmosphere without significant absorption
  • Experiences scattering by air molecules and dust particles, resulting in blue skies and reddish sunsets (Rayleigh scattering)
• Ultraviolet
  • Largely absorbed by the ozone layer in Earth's stratosphere
  • Ozone layer acts as a protective shield, preventing harmful UV radiation from reaching Earth's surface and damaging life
• X-rays and Gamma rays
  • Completely absorbed by Earth's atmosphere at high altitudes
  • Studying cosmic X-ray and gamma-ray sources requires space-based observatories (Chandra X-ray Observatory, Fermi Gamma-ray Space Telescope)

Temperature and light emission

• Blackbody radiation
  • Describes an idealized object that perfectly absorbs all incoming light and emits a spectrum dependent on its temperature
  • Real objects approximate blackbody behavior to varying degrees (stars, planets)
• Wien's displacement law
  • Relates an object's temperature to the peak wavelength of its emitted radiation: $\lambda_{max} = \frac{2.898 \times 10^{-3}}{T}$
  • Hotter objects emit their peak radiation at shorter wavelengths (blue stars vs. red stars)
• Stefan-Boltzmann law
  • Quantifies the total energy emitted per unit surface area of an object: $L = 4\pi R^2 \sigma T^4$
  • Hotter objects emit significantly more energy across all wavelengths (luminosity)
• Examples
  • The Sun, with a surface temperature of ~5800 K, emits most of its radiation in the visible light range
  • Earth, with an average surface temperature of ~300 K, emits most of its radiation in the infrared
  • Human bodies, with skin temperatures around 310 K, also emit primarily in the infrared (thermal imaging)

Properties of Electromagnetic Waves

• Electromagnetic waves are transverse waves that propagate through space, carrying energy and information
• All electromagnetic waves travel at the speed of light in a vacuum, approximately 3 x 10^8 m/s
• Amplitude refers to the maximum displacement of the wave from its equilibrium position, related to the wave's intensity
• Polarization describes the orientation of the electric field oscillations in an electromagnetic wave
• Interference occurs when two or more waves interact, resulting in constructive or destructive combinations
• Diffraction is the bending of waves around obstacles or through openings, allowing light to spread into regions of geometric shadow
• Refraction is the change in direction of a wave as it passes from one medium to another due to a change in its speed