Red-Shift and the Doppler Effect

This lesson covers the Doppler effect and red-shift of light from distant galaxies — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand what the Doppler effect is, how it applies to light from galaxies, and what red-shift tells us about the motion of the universe.

---

The Doppler Effect

The Doppler effect is the apparent change in the wavelength (and frequency) of a wave when there is relative motion between the source and the observer.

The Doppler Effect with Sound

You have probably experienced the Doppler effect with sound:

• When an ambulance approaches you, its siren sounds higher pitched (higher frequency, shorter wavelength).
• When the ambulance moves away from you, its siren sounds lower pitched (lower frequency, longer wavelength).

Why does this happen?

• As the source moves towards you, the sound waves are compressed — the wavefronts are closer together, so the wavelength is shorter and the frequency is higher.
• As the source moves away from you, the sound waves are stretched — the wavefronts are further apart, so the wavelength is longer and the frequency is lower.
• The actual frequency of the siren has not changed — it only appears to change because of the relative motion.

Direction of Motion	Wavelength	Frequency	Pitch (Sound)
Source approaching observer	Shorter (compressed)	Higher	Higher pitch
Source moving away from observer	Longer (stretched)	Lower	Lower pitch

Exam Tip: The Doppler effect occurs with ALL types of waves — sound, light, and other electromagnetic waves. The key principle is the same: approaching = shorter wavelength, moving away = longer wavelength.

---

The Doppler Effect with Light

The Doppler effect also applies to light (and all electromagnetic radiation):

Blue-Shift

• If a light source is moving towards the observer, its light waves are compressed.
• The wavelength appears shorter — shifted towards the blue end of the visible spectrum.
• This is called blue-shift.

Red-Shift

• If a light source is moving away from the observer, its light waves are stretched.
• The wavelength appears longer — shifted towards the red end of the visible spectrum.
• This is called red-shift.

Direction of Motion	Effect on Light	Name
Source moving towards observer	Wavelength shortened → shifted to blue end	Blue-shift
Source moving away from observer	Wavelength lengthened → shifted to red end	Red-shift

The Visible Spectrum

For reference, the visible spectrum runs from:

Colour	Wavelength (approximate)
Violet	~400 nm (shortest visible wavelength)
Blue	~470 nm
Green	~530 nm
Yellow	~580 nm
Orange	~600 nm
Red	~700 nm (longest visible wavelength)

So red-shift means the wavelength increases (shifts towards the longer, red end), and blue-shift means the wavelength decreases (shifts towards the shorter, blue end).

Exam Tip: Red-shift does not mean the light actually turns red. It means the wavelength is shifted to longer wavelengths. The light from a distant galaxy may still appear white or blue — but its spectral lines will be shifted compared to where they would be if the galaxy were stationary.

---

Observing Red-Shift in Galaxies

When astronomers observe the light from distant galaxies, they find that the spectral lines (specific wavelengths absorbed or emitted by elements) are shifted towards the red end of the spectrum compared to their laboratory positions.

What This Tells Us

• Almost all distant galaxies show red-shift — their light has been shifted to longer wavelengths.
• This means almost all galaxies are moving away from us (receding).
• The light from nearby galaxies shows a small red-shift.
• The light from distant galaxies shows a large red-shift.

The Key Relationship

The further away a galaxy is, the greater its red-shift.

This means:

• The further away a galaxy is, the faster it is moving away from us.
• This relationship is fundamental to our understanding of the expanding universe (covered in the next lesson).

How Red-Shift Is Measured

Astronomers compare the absorption spectrum (or emission spectrum) of a distant galaxy with the known spectrum of the same elements measured in a laboratory on Earth:

Feature	Laboratory Spectrum	Distant Galaxy Spectrum
Spectral line positions	At known, fixed wavelengths	Same pattern, but shifted to longer wavelengths
Amount of shift	None (reference)	Depends on how fast the galaxy is receding

The pattern of spectral lines is the same (allowing identification of the elements), but all lines are shifted by the same proportional amount towards the red end.

---

What About Blue-Shift?

A few galaxies show blue-shift rather than red-shift — they are moving towards us:

• The Andromeda galaxy (our nearest large galaxy) is blue-shifted — it is approaching the Milky Way at about 110 km/s.
• In about 4.5 billion years, the Andromeda galaxy and the Milky Way are expected to collide and merge.
• However, the vast majority of galaxies show red-shift — the overall pattern is that the universe is expanding.

Exam Tip: If asked whether all galaxies are red-shifted, the answer is no — a few nearby galaxies (like Andromeda) show blue-shift because their local gravitational attraction overcomes the general expansion. But almost all galaxies, especially distant ones, are red-shifted.

---

Worked Example

Question: Light from a distant galaxy has a wavelength that is 5% longer than expected. What does this tell us about the galaxy?

Answer:

• The wavelength is longer than expected → the light has been red-shifted.
• Red-shift means the galaxy is moving away from us (receding).
• A 5% increase in wavelength indicates the galaxy is receding at a significant speed.

---

Linking Red-Shift to the Electromagnetic Spectrum