2.2.3 EM spectrum in Cosmology

2.2.3 EM spectrum in Cosmology: List the regions of the electromagnetic spectrum in order and explain how different regions help astronomers study the universe.

Visible light is only a narrow part of a much wider family of waves.

Order

In order of increasing frequency the regions are radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.

Wavelength

Wavelength decreases as frequency increases, so radio waves are the longest and gamma rays the shortest.

Common speed

All electromagnetic waves travel at 3.00 × 10⁸ m/s in a vacuum, so they cross space at the same speed.

Wave equation

Speed, frequency and wavelength are linked by v = fλ for every region of the spectrum.

Region	Typical use in astronomy
Radio	Detecting cool gas and distant galaxies
Microwave	Studying the cosmic microwave background
Infrared	Seeing through dust and observing cooler stars
Visible	Imaging stars and galaxies through telescopes
Ultraviolet	Observing hot young stars
X-ray	Studying very hot gas and black hole surroundings
Gamma	Detecting the most energetic events

Objects give out different radiation, so using many regions builds a fuller picture.

Observing the Sky

Different information

A galaxy can look quite different in radio waves, visible light and X-rays, revealing separate processes.

Telescope types

Radio dishes, optical telescopes and X-ray detectors are each designed for their own region.

The atmosphere

The atmosphere blocks most ultraviolet, X-ray and gamma radiation, so those telescopes must orbit above it.

Clear windows

Visible light and some radio waves pass through the atmosphere, so ground-based telescopes can use them.

Careful measurement of received radiation supports ideas about the universe.

Using the Spectrum as Evidence

Red-shift

Wavelengths from receding galaxies appear longer, so visible features move towards the red end.

Microwave background

Microwave detectors find radiation from all directions that supports the Big Bang theory.

Detecting the unseen

Infrared and radio observations reveal objects hidden by dust that visible light cannot show.

Comparing sources

Combining images from different regions helps astronomers decide how hot or active an object is.

Question: A radio wave from space has a frequency of 1.00 × 10⁸ Hz. Calculate its wavelength.

Use v = fλ, so λ = v ÷ f.
v = 3.00 × 10⁸ m/s and f = 1.00 × 10⁸ Hz.
λ = 3.00 × 10⁸ ÷ 1.00 × 10⁸ = 3.00 m.

Answer: The wavelength is 3.00 m.

2.2.3 – Key Points for the Exam

Electromagnetic spectrum: The family of waves that all travel at the same speed in a vacuum.
Wavelength: The distance between a point on a wave and the matching point on the next wave.
Frequency: The number of waves produced or passing a point each second, in hertz.
Space telescope: A telescope in orbit that avoids absorption by the atmosphere.

Common Mistakes

Putting the regions in the wrong order, especially mixing up infrared and ultraviolet.
Saying radio waves travel slower than light; every region travels at 3.00 × 10⁸ m/s in a vacuum.
Thinking all telescopes can work on the ground; X-rays and gamma rays are absorbed by the atmosphere.
Confusing high frequency with long wavelength; they are opposites.