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Astronomy & space · Grades 8–12

Light, spectra and astronomical evidence

Radio, infrared, visible, ultraviolet, X-rays and gamma rays are electromagnetic radiation. Astronomy uses more than the light visible to human eyes.

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Prerequisites

Wave frequency, graphs and ratios.

Study notes

Electromagnetic spectrum

Radio, infrared, visible, ultraviolet, X-rays and gamma rays are electromagnetic radiation. Astronomy uses more than the light visible to human eyes.

Wave relation

In vacuum, frequency and wavelength satisfy c=fλ. Shorter wavelength therefore means higher frequency at fixed propagation speed.

Spectra

Spectral features can constrain composition and physical conditions. A spectrum is evidence interpreted through models, not a labelled photograph of individual atoms.

Lookback

Light takes time to travel. Seeing a distant object means receiving information emitted earlier, with cosmological interpretation requiring care at very large distances.

Worked examples

Which is electromagnetic radiation used in astronomy?

Radio waves are electromagnetic, like visible light.

Radio waves

Using c=3×10^8 m/s and wavelength 3 m, find frequency in MHz.

f=c/λ=10^8 Hz=100 MHz.

100 MHz

Check your understanding

  1. At fixed vacuum speed, reducing wavelength increases what?

    1. Vacuum light speed
    2. Time per cycle
    3. Mass of the observer
    4. Frequency

  2. A signal travels 600000 km at 300000 km/s. Find travel time.

  3. An absorption feature indicates what most directly?

    1. Distance is exactly zero
    2. Some wavelengths are removed relative to neighbouring light
    3. The source has no temperature
    4. Every atom has been photographed

  4. Why do astronomers combine observations from different wavelength bands?

Answers and marking guidance

Answers and marking notes
  1. Frequency

    Frequency and wavelength are inversely related.

  2. 2 s

    Time equals distance divided by speed.

  3. Some wavelengths are removed relative to neighbouring light

    Interpretation of the feature requires a model of absorbing material.

  4. Different processes, temperatures or obscuring material affect bands differently. Combined observations can constrain models more strongly than a single band.

    Credit complementarity rather than claiming more images guarantee truth.

Further reading and review references

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