Light and the electromagnetic spectrum
Almost everything we know about the universe beyond Earth comes from light. Understanding light is the key to decoding the cosmos.
Light as a wave
Light is an electromagnetic wave, travelling through space at about 300,000 km/s (c). Wavelength (lambda) is the distance between crests; frequency (f) is how many crests pass per second. They are related by \(c = \lambda f\): shorter wavelengths have higher frequencies.
c = \lambda fLight as photons
Light also comes in packets called photons. Each photon's energy is \(E = hf\), where h is Planck's constant. Shorter-wavelength (higher-frequency) photons carry more energy: an ultraviolet photon carries more energy than a red one, which is why UV light can damage skin and DNA.
E = hfThe electromagnetic spectrum
From longest to shortest wavelength, the spectrum runs: radio waves, microwaves, infrared, visible light (red to violet, about 400-700 nanometres), ultraviolet, X-rays, and gamma rays. Visible light is only a tiny slice. Different objects emit different kinds of light, so astronomers observe across the whole spectrum.
Earth's atmosphere as a filter
Earth's atmosphere blocks most wavelengths. Visible light and many radio waves reach the ground, but most infrared, ultraviolet, X-rays, and gamma rays are absorbed. That is why X-ray and gamma-ray telescopes must be in space, and infrared telescopes are placed on high, dry mountains or in space.
Light travel time
Because light takes time to travel, we see distant objects as they were in the past. Sunlight takes about 8.3 minutes to reach Earth; light from the nearest star system, Alpha Centauri, takes about 4.4 years. A light-year, the distance light travels in a year, is about 9.46 trillion km. Looking far away means looking back in time.
Thermal radiation
All objects with temperature emit thermal (blackbody) radiation. Hotter objects emit more energy at every wavelength and peak at shorter wavelengths. Wien's law says peak wavelength is inversely proportional to temperature: a cool red star is about 3,000 K, the Sun about 5,800 K, and a hot blue star 20,000 K or more.
Brightness and the inverse-square law
The brightness of a light source decreases with the square of distance: move twice as far, and it appears one-quarter as bright. Astronomers use this to estimate distances when they know how much light an object actually emits (its luminosity).
Worked example
Star A has a surface temperature of 3,000 K and Star B of 6,000 K. By Wien's law, B's peak wavelength is half of A's, shifting it from infrared-red toward yellow-green. By the Stefan-Boltzmann law (energy per area proportional to \(T^4\)), each square metre of B emits \(2^4 = 16\) times more energy than A.
Key takeaways
Light travels at about 300,000 km/s; c = lambda f. Photon energy E = hf rises with frequency. The spectrum runs from radio to gamma rays; visible light is a narrow band. The atmosphere blocks most non-visible light, requiring space telescopes. Looking far means looking back in time; a light-year is about 9.46 trillion km. Hotter objects peak at shorter wavelengths (Wien's law) and emit far more energy. Brightness falls with distance squared.
Flashcard
About 300,000 km/s
Check your understanding
Why are X-ray telescopes placed in space rather than on the ground?