More Than Meets the Eye
The light our eyes can see, visible light, is just a tiny fraction of a much broader range of radiation called the electromagnetic (EM) spectrum. All forms of EM radiation are waves of oscillating electric and magnetic fields that travel at the speed of light (c).
Regions of the EM Spectrum
The spectrum is organized by wavelength or frequency. From longest wavelength to shortest wavelength (or lowest frequency/energy to highest), the order is:
1.Radio Waves: Longest wavelengths. Used in radio and television communication.
2.Microwaves: Used in radar and microwave ovens. The Cosmic Microwave Background is in this range.
3.Infrared (IR): We feel this as heat. Used in night vision and thermal imaging.
4.Visible Light: The narrow band our eyes can detect. The order of colors from long to short wavelength is ROY G BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
5.Ultraviolet (UV): Can cause sunburn. The ozone layer protects us from most of the Sun's UV light.
6.X-rays: Used in medical imaging. Can penetrate soft tissues.
7.Gamma Rays: Shortest wavelengths and highest energy. Emitted by the most energetic events in the universe, like supernovae and nuclear reactions.
Key Relationships
For all EM waves, the following relationships hold:
Speed: All travel at the speed of light, c (≈ 3 x 10⁸ m/s).
Wavelength and Frequency: They are inversely proportional. Longer wavelength means lower frequency. The relationship is c = fλ.
Energy and Frequency: They are directly proportional. Higher frequency means higher energy. The relationship is E = hf, where h is Planck's constant.
Combining these, energy is inversely proportional to wavelength. Short-wavelength radiation like gamma rays is very high-energy, while long-wavelength radio waves are low-energy.
Multi-wavelength Astronomy
Earth's atmosphere is transparent to visible light and radio waves, which is why ground-based astronomy was historically limited to these 'windows'. However, the atmosphere blocks most infrared, ultraviolet, X-ray, and gamma-ray radiation.
To observe the universe in these other wavelengths, astronomers must use space-based telescopes (like the Hubble, Chandra X-ray Observatory, and James Webb Space Telescope).
Observing in different wavelengths reveals different physical processes. Hot, violent events emit X-rays and gamma rays. Cool dust clouds glow in the infrared. Looking across the entire spectrum gives a much more complete picture of the universe.