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Astronomy & space · lesson 3 of 4

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.

Grades 8–128 practice questionsNotes + worked examples
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Learn the ideas

Prerequisites: Wave frequency, graphs and ratios.

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

Example 1

Which is electromagnetic radiation used in astronomy?

Radio waves are electromagnetic, like visible light.

Radio waves

Example 2

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

8 questions

Decimals or fractions are accepted for numeric answers. Proofs and written responses use sample answers and require human marking. This is practice, not a protected examination.

1 Which is electromagnetic radiation used in astronomy?

Show answer and explanation

Radio waves

Radio waves are electromagnetic, like visible light.

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

Show answer and explanation

100 MHz

f=c/λ=10^8 Hz=100 MHz.

3 At fixed vacuum speed, reducing wavelength increases what?

Show answer and explanation

Frequency

Frequency and wavelength are inversely related.

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

Show answer and explanation

2 s

Time equals distance divided by speed.

5 An absorption feature indicates what most directly?

Show answer and explanation

Some wavelengths are removed relative to neighbouring light

Interpretation of the feature requires a model of absorbing material.

6 Why do astronomers combine observations from different wavelength bands?

Sample answer and marking guidance

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.

7 Distinguish an observed spectral line from an inferred chemical identification.

Sample answer and marking guidance

The line is a measured feature; identifying a species uses reference transitions and physical models, with uncertainty and possible blends considered.

Credit observation versus model-based inference.

8 When we observe light from a distant star, are we seeing information from its present local moment?

Show answer and explanation

Not necessarily; the light was emitted earlier

All electromagnetic signals propagate with finite speed.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.