5.2 The Electromagnetic Spectrum

The electromagnetic spectrum covers the full range of energy waves, from long radio waves to short gamma rays. Each type of radiation interacts differently with matter and with Earth's atmosphere, which directly affects how astronomers observe the universe. This topic also connects to everyday phenomena like why the sky is blue, how microwaves heat food, and why you get sunburned.

The Electromagnetic Spectrum

Bands of the Electromagnetic Spectrum

All electromagnetic radiation is the same fundamental thing: oscillating electric and magnetic fields traveling through space. What distinguishes one type from another is wavelength (and therefore frequency and energy). Moving from longest wavelength to shortest:

• Radio waves have the longest wavelengths (meters to kilometers) and the lowest energy. They're used for transmitting radio and television signals over long distances.
• Microwaves have shorter wavelengths (millimeters to centimeters) and higher energy than radio waves. They're used in microwave ovens and radar systems.
• Infrared (IR) has shorter wavelengths still (roughly 700 nm to 1 mm). Every object with a temperature above absolute zero emits some infrared radiation. Applications include night vision devices and weather satellites.
• Visible light is the narrow band of wavelengths (about 400–700 nm) that human eyes can detect. Colors range from red (longest visible wavelength) to violet (shortest), following the familiar order ROYGBIV.
• Ultraviolet (UV) has shorter wavelengths and higher energy than visible light (roughly 10–400 nm). UV radiation can damage living tissue (sunburns) and is used in sterilization processes like water purification.
• X-rays have even shorter wavelengths (roughly 0.01–10 nm) and enough energy to pass through soft tissue, which is why they're used in medical imaging and airport security scanners.
• Gamma rays have the shortest wavelengths and highest energy in the spectrum. They're produced by radioactive decay and extreme cosmic events like supernovae and pulsars.

The key relationship to remember: as wavelength gets shorter, frequency and energy both increase. This is captured by $E = h \nu$, where $h$ is Planck's constant and $\nu$ is frequency.

Electromagnetic Spectrum and Earth's Atmosphere

Not all wavelengths reach Earth's surface. The atmosphere is transparent to some bands and opaque to others, and this has huge consequences for astronomy.

• Radio waves pass through the atmosphere with minimal absorption. This is why ground-based radio telescopes work well.
• Microwaves largely pass through, though atmospheric water vapor and oxygen absorb some of them.
• Infrared is partially absorbed, mainly by water vapor and carbon dioxide. This absorption is central to the greenhouse effect: atmospheric gases absorb IR radiation emitted by Earth's surface and re-emit it in all directions, warming the planet.
• Visible light transmits through the atmosphere without much absorption. It does get scattered by air molecules and dust particles, though. Rayleigh scattering preferentially scatters shorter (blue) wavelengths, which is why the sky looks blue during the day and sunsets appear reddish (the blue light has been scattered away along the longer path through the atmosphere).
• Ultraviolet is largely absorbed by the ozone layer in the stratosphere. This layer acts as a shield, preventing most harmful UV from reaching the surface.
• X-rays and gamma rays are completely absorbed at high altitudes. To study cosmic sources of X-rays and gamma rays, astronomers must use space-based observatories like the Chandra X-ray Observatory and the Fermi Gamma-ray Space Telescope.

The bands that do reach the ground create what astronomers call atmospheric windows. The two main windows are in visible light and radio waves, which is why optical and radio telescopes can operate from Earth's surface.

Temperature and Light Emission

Every object with a temperature above absolute zero emits electromagnetic radiation. The pattern of that emission depends on the object's temperature, and understanding this connection is one of the most powerful tools in astronomy.

• Blackbody radiation describes an idealized object that perfectly absorbs all incoming light and emits a smooth, continuous spectrum determined entirely by its temperature. Stars approximate blackbody behavior quite well.
• Wien's displacement law tells you where the peak of that emission falls:

$\lambda_{\text{max}} = \frac{2.898 \times 10^{-3}}{T}$

where $\lambda_{\text{max}}$ is the peak wavelength in meters and $T$ is temperature in Kelvin. Hotter objects peak at shorter wavelengths. That's why hot stars appear blue and cooler stars appear red.

• The Stefan-Boltzmann law tells you how much total energy an object radiates:

$L = 4\pi R^2 \sigma T^4$

where $L$ is luminosity, $R$ is the object's radius, $\sigma$ is the Stefan-Boltzmann constant, and $T$ is surface temperature. Because of that $T^4$ term, even a modest increase in temperature produces a dramatic increase in energy output.

Some concrete examples that show these laws in action:

• The Sun (surface temperature ~5800 K) peaks in visible light, which is no coincidence: human eyes evolved to be most sensitive to the wavelengths our star emits most strongly.
• Earth (average surface temperature ~300 K) peaks in the infrared, which is why the greenhouse effect involves IR absorption.
• Human bodies (skin temperature ~310 K) also emit primarily in the infrared, which is the basis for thermal imaging cameras.

Properties of Electromagnetic Waves

Electromagnetic waves are transverse waves, meaning the electric and magnetic fields oscillate perpendicular to the direction the wave travels. A few core properties apply to all of them:

• Speed: All electromagnetic waves travel at the speed of light in a vacuum, approximately $3 \times 10^8$ m/s. They slow down when passing through matter.
• Amplitude is the maximum displacement of the wave from its resting position. Higher amplitude means greater intensity (brighter light, stronger signal).
• Polarization describes the orientation of the electric field oscillations. Unpolarized light has electric fields vibrating in all directions; polarized light vibrates in just one plane (this is how polarized sunglasses reduce glare).
• Interference occurs when two or more waves overlap. If their peaks align, they combine to make a stronger wave (constructive interference). If a peak meets a trough, they cancel out (destructive interference).
• Diffraction is the bending of waves around obstacles or through narrow openings. It allows light to spread into regions that would otherwise be in shadow, and it sets limits on telescope resolution.
• Refraction is the change in direction of a wave as it passes from one medium to another (like from air into glass) due to a change in speed. This is how lenses focus light.