Astronomy & Space · Unit 15

Light, Spectra and Distances

The spectrum, spectroscopy, parallax, standard candles and redshift

Astronomy is unusual among sciences in that you cannot touch the thing you study. Almost everything known about a star comes from analysing its light, and this unit is about how.

It covers the electromagnetic spectrum, spectroscopy and what spectral lines reveal, parallax, standard candles, and redshift.

This unit breaks down into 21 short steps and 120 questions, starting at difficulty 3 and building to 4. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 21
Questions: 120
Difficulty: 3-4

What this unit covers

Parallax
Spectroscopy
Standard Candles
The Electromagnetic Spectrum
Redshift

Where this fits

Needs The Sun and Stars. Underpins Cosmology.

Where people slip

Cosmological redshift is not a Doppler shift. The light is stretched by the expansion of space it travels through rather than by the source moving away through space.

How the unit is structured

Light, Spectra and Distances runs as 21 short steps that unlock in order. 15 are practice rounds and 6 are challenge rounds that pull together everything before them. Questions start at difficulty 3 and climb to 4 as you progress.

Step 1 · easierStep 21 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Parallax

Build the sentenceLevel 3
1. Build a true sentence about how parallax works.
Answer: Nearby stars shift as Earth orbits the Sun
This tiny shift is parallax, used to measure star distances.
Choose all that applyLevel 3
2. Select all that are true about parallax.
- It measures distances to nearby starscorrect
- Nearer stars show a bigger shiftcorrect
- It uses Earth's orbit as a baselinecorrect
- It measures a star's temperature
Parallax is a geometric distance method; temperature comes from other methods.
Fill the blankLevel 3
3. As Earth orbits the Sun, a nearby star seems to shift against the ____ background stars.
- distantcorrect
- nearby
- brighter
- colored
Only far-off stars stay fixed enough to act as a background for the shift.
Guess the numberLevel 3
4. About how many light-years is one parsec?
Answer: 3.26 light-years
One parsec equals roughly 3.26 light-years.
Fact or fibLevel 4
5. All stars have parallax angles smaller than one arcsecond.
Answer: True
Even the nearest star, Proxima Centauri, shifts by less than one arcsecond.
Match the pairsLevel 4
6. Match each parallax angle to the distance it gives.
Answer: Parallax of 1 arcsecond = 1 parsec; Parallax of 0.5 arcseconds = 2 parsecs; Parallax of 0.1 arcseconds = 10 parsecs
distance in parsecs is one over the parallax in arcseconds.

Redshift

Build the sentenceLevel 3
7. Build a true sentence about redshift.
Answer: Redshift stretches light to longer wavelengths
Redshift lengthens light waves as a source moves away.
Fact or fibLevel 3
8. A larger redshift generally means a galaxy is farther away.
Answer: True
In the expanding universe, greater redshift goes with greater distance.
Fill the blankLevel 3
9. Hubble found that most galaxies are ____, which means the universe is expanding.
- redshiftedcorrect
- blueshifted
- cooling
- shrinking
Their light is stretched, showing they are moving away as space expands.
Multiple choiceLevel 3
10. When a galaxy is moving away from us, its light is:
- Redshiftedcorrect
- Blueshifted
- Made brighter
- Turned into radio
Motion away stretches the light waves to longer, redder wavelengths, called redshift.
Choose all that applyLevel 4
11. Select all that are true about redshift.
- It stretches light to longer wavelengthscorrect
- Larger redshift means faster recessioncorrect
- It supports an expanding universecorrect
- It only affects radio waves
Redshift affects all wavelengths and points to cosmic expansion.
Put in orderLevel 4
12. Put these steps in the order that led to the idea of an expanding universe.
Answer: Measure galaxy distances with Cepheids -> Measure galaxy redshifts -> Find that speed rises with distance -> Conclude the universe is expanding
Hubble combined galaxy distances and redshifts to reveal cosmic expansion.

Spectroscopy

Fact or fibLevel 3
13. Each chemical element makes its own unique pattern of spectral lines.
Answer: True
These line patterns act like fingerprints that reveal what a star is made of.
Fill the blankLevel 3
14. A hot, thin gas glows with bright ____ lines at specific wavelengths.
- emissioncorrect
- absorption
- shadow
- radio
Hot thin gas emits light only at its own special wavelengths, making emission lines.
Match the pairsLevel 3
15. Match each spectrum type to its description.
Answer: Emission lines = Bright lines from hot gas; Absorption lines = Dark gaps in a spectrum; Continuous spectrum = Smooth rainbow from a hot solid
The three basic spectrum types come from Kirchhoff's laws.
Multiple choiceLevel 3
16. When white light passes through a prism, what does it produce?
- A spectrum of colorscorrect
- A single beam of white
- A dark shadow
- A radio signal
A prism bends different colors by different amounts, spreading white light into a spectrum.
Odd one outLevel 3
17. Which one can spectroscopy NOT directly reveal about a star?
- Its exact age in yearscorrect
- Its chemical makeup
- Its temperature
- Its motion toward or away
Spectra show composition, temperature, and motion, but not a star's age directly.
Choose all that applyLevel 4
18. Select all that spectroscopy can tell us about a star.
- Its chemical compositioncorrect
- Its temperaturecorrect
- Its motion toward or awaycorrect
- Its exact number of planets
Spectra reveal composition, temperature, and motion, but not a planet count.

Standard Candles

Build the sentenceLevel 3
19. Build a true sentence about Cepheid stars.
Answer: Cepheid stars help measure cosmic distances
Their steady pulses reveal true brightness and therefore distance.
Fact or fibLevel 3
20. The farther away a light source is, the fainter it looks.
Answer: True
Brightness fades with distance, which is why standard candles reveal how far away things are.
Fill the blankLevel 3
21. Apparent brightness fades with the ____ of the distance.
- squarecorrect
- cube
- root
- double
Light spreads out, so brightness drops as the square of the distance (the inverse-square law).
Match the pairsLevel 3
22. Match each term to its meaning.
Answer: Cepheid variable = Period-luminosity relation; Type Ia supernova = Consistent peak brightness; Standard candle = Known true brightness
Each is a way astronomers pin down true brightness to measure distance.
Multiple choiceLevel 3
23. A standard candle is an object whose known ____ lets us work out its distance.
- True brightnesscorrect
- Exact age
- Chemical color
- Spin speed
If you know an object's real brightness, its faintness tells you how far away it is.
Odd one outLevel 3
24. Which of these is NOT a standard candle?
- A parallax anglecorrect
- A Cepheid variable
- A Type Ia supernova
- An RR Lyrae star
Parallax is a separate distance method, not a standard-candle light source.

The Electromagnetic Spectrum

Choose all that applyLevel 3
25. Select all that are forms of electromagnetic radiation.
- X-rayscorrect
- Microwavescorrect
- Gamma rayscorrect
- Sound
X-rays, microwaves, and gamma rays are all light; sound is not.
Fact or fibLevel 3
26. Radio waves are more dangerous to people than gamma rays.
Answer: False
Gamma rays carry far more energy and are the dangerous ones; radio waves are low-energy.
Fill the blankLevel 3
27. Of the visible colors, ____ light has the longest wavelength.
- redcorrect
- violet
- blue
- green
Red is at the long-wavelength end of the visible rainbow.
Match the pairsLevel 3
28. Match each type of light to its property.
Answer: Radio waves = Longest wavelength; Gamma rays = Highest energy; Infrared = Felt as heat
Radio has the longest waves, gamma the most energy, and infrared is warmth we can feel.
Multiple choiceLevel 3
29. Which type of electromagnetic radiation carries the most energy?
- Gamma rayscorrect
- Radio waves
- Microwaves
- Infrared
Gamma rays have the shortest wavelength and the highest energy of all light.
Odd one outLevel 3
30. Which one is NOT a type of electromagnetic radiation?
- Sound wavescorrect
- X-rays
- Radio waves
- Infrared
Sound is a vibration of matter, not electromagnetic light.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 14 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sort into groups
Spell it
True or false
Type the answer

Practise Light, Spectra and Distances

120 questions across 21 steps. Start with step one and crawl at your own pace.

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Read about Light, Spectra and Distances

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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