Observing with Light Waves
Describe the electromagnetic spectrum from radio waves to gamma rays. Give examples of what radio, microwave, infrared, visible, ultraviolet and X-ray observations reveal. Explain atmospheric limits, direct and indirect gamma-ray observations, and the wavelength choices of Hubble and Webb.
What a learner can do afterwards
- Lists at least four regions of the EM spectrum and gives a specific astronomical object or phenomenon observed in each
- Explains why direct astronomical X-ray and gamma-ray detectors operate above the absorbing atmosphere, and why space provides access to infrared bands blocked from the ground
- Describes the James Webb Space Telescope or Hubble and explains which part of the spectrum each primarily observes and why that was chosen
1 · Read
Astronomers use starlight to find planets. They also study light our eyes cannot detect. The electromagnetic spectrum includes all these kinds of light. Radio waves have the longest wavelengths; gamma rays have the shortest. Wavelength is the distance between neighbouring wave crests. Frequency counts cycles passing each second. In a vacuum, all electromagnetic waves have the same speed, so higher frequency goes with shorter wavelength.
Radio observations can map cold gas clouds where stars may form. Microwaves reveal the cosmic microwave background: light released in the early universe and stretched during expansion. X-rays reveal extremely hot gas, including gas near black holes. Infrared reveals warm dust and forming stars. Visible light shows stars and galaxies. Ultraviolet helps trace hot young stars.
Visible light and some radio and infrared wavelengths pass through Earth's air. It absorbs incoming astronomical X-rays and gamma rays. Direct detectors for those rays operate above the absorbing air. Ground gamma-ray observatories instead detect secondary signals made when gamma rays interact with air. Space also provides access to infrared bands blocked from the ground.
Hubble observes ultraviolet, visible and some near-infrared light. Being in space lets it collect ultraviolet wavelengths blocked by air and avoid atmospheric blurring. Images can use assigned colours to display measurements, so they need not match what your eyes would see.
Webb observes infrared. Some infrared wavelengths pass through dust more readily than visible light, revealing forming stars. Expansion stretches light from very distant galaxies to longer wavelengths. Light emitted as visible light can reach us in the infrared. Webb uses infrared measurements to study early galaxies.
This example shows one possible wavelength change during travel through expanding space.
Different wavelengths reveal different objects and processes. Earth's air passes some bands and blocks others, so astronomers use both ground and space observatories.
Companion readings and flashcards written for this learning dataset. Linked video: Khan Academy, The electromagnetic spectrum.
More to read
Describing light our eyes cannot see
A telescope does not have to collect visible light. Its detector can be built for another region of the electromagnetic spectrum. Astronomers commonly list the regions as radio, microwave, infrared, visible, ultraviolet, X-ray and gamma-ray. This order runs toward shorter wavelengths. The regions are not equal-sized divisions.
Wavelength is the distance between neighbouring wave crests. Frequency counts the cycles passing a point each second. In a vacuum, every electromagnetic wave has the same speed. Shorter wavelengths therefore go with higher frequencies.
An observation at invisible wavelengths must be displayed in visible colours for people to view it as an image. Those assigned colours can show differences in the measurements. They do not establish what human eyes would see at the object.
What reaches the detector?
Before choosing a telescope site, ask whether its target radiation reaches that site. Visible light and some radio and infrared wavelengths pass through Earth's air. Other bands are absorbed. Incoming astronomical X-rays do not reach a mountaintop as a usable direct signal.
Gamma-ray observers use two approaches. A detector in space can receive incoming gamma rays directly. A ground observatory can instead detect secondary signals produced when gamma rays interact with the air. The ground method uses the atmosphere as part of the measurement.
Infrared telescopes can work from the ground in bands that pass through the air. Space offers access to other bands and reduces interference from warm surroundings. It also avoids atmospheric blurring. These benefits do not mean every telescope belongs in space, where building and servicing instruments is difficult.
Choosing observations for a question
An astronomer studying a region of star formation may need several measurements. Radio observations can map cold gas. Infrared observations can reveal warm dust and some forming stars hidden by dust. Visible light shows stars and galaxies, while ultraviolet helps trace hot young stars. X-rays can reveal extremely hot gas near black holes or in supernova remnants.
These are examples, not rules that each object emits only one kind of light. Comparing measurements can reveal different parts or processes within the same object.
Hubble observes ultraviolet, visible and some near-infrared light. Webb observes infrared. Some infrared passes through dust more readily than visible light. Infrared also lets Webb study light from distant galaxies stretched during expansion. Microwave instruments study another signal: the cosmic microwave background, light released in the early universe rather than at its first instant.
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2 · Watch
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Where it sits
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.