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8.3.3The Periodic Table

Galactic Red-shift — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 23 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5What Red-shift Shows

The light from distant galaxies gives direct evidence of their motion.

Red-shift
  • Red-shift An increase in the observed wavelength of light from a source moving away from the observer, shifting the spectrum towards red.
Distant galaxies
  • Distant galaxies Light from most distant galaxies is red-shifted, which means those galaxies are moving away from Earth.
Spectral lines
  • Spectral lines Known spectral lines of elements appear at longer wavelengths in the light from a receding galaxy.
Frequency
  • Frequency A red-shift corresponds to a decrease in frequency, because wavelength and frequency are inversely related for light.
Expanding Universe
  • Expanding Universe Because most distant galaxies show red-shift, the Universe as a whole is inferred to be expanding.

Part 2 of 5Distance and Recession Speed

1
Astronomers measure the observed wavelengths of known spectral lines in light from a galaxy and compare them with laboratory values.
2
The change in wavelength gives the recession speed of the galaxy, using Δλ / λ₀ = v / c.
3
Galaxies that are further away show a greater red-shift, which means they are receding faster.
4
A graph of recession speed against distance is a straight line through the origin, so the relationship is directly proportional.
5
This linear relationship supports the idea that space has been expanding ever since the Big Bang.

Part 3 of 5Interpreting the Evidence

Interpreting the Evidence
Red-shift against distance
  • Red-shift against distance A graph of red-shift against distance rises steadily, showing that greater distance gives greater red-shift.
Cause of red-shift
  • Cause of red-shift Red-shift is caused by the expansion of space stretching the light waves as they travel towards Earth.
Nearby exceptions
  • Nearby exceptions Some nearby galaxies may show a blueshift, because their own motion can outweigh the effect of expansion.
Support for the Big Bang
  • Support for the Big Bang Galactic red-shift together with the cosmic microwave background supports the Big Bang as the origin of the Universe.

Part 4 of 5Worked Example

Question

  • A hydrogen spectral line from a distant galaxy has a reference wavelength of 486 nm.
  • The galaxy is receding at a speed of 1.5 × 10⁶ m/s.
  • Calculate the observed wavelength of this line.
  • (c = 3.0 × 10⁸ m/s)

  1. Step 1: Rearrange Δλ / λ₀ = v / c to give Δλ = λ₀ × v / c.
  2. Step 2: Substitute: Δλ = 486 × (1.5 × 10⁶) / (3.0 × 10⁸) = 486 × 0.005 = 2.43 nm.
  3. Step 3: The observed wavelength is λ = λ₀ + Δλ = 486 + 2.43 = 488.43 nm.

Answer The observed wavelength is about 488 nm, which is red-shifted compared with the reference value of 486 nm.

Part 5 of 5Exam Checklist & Tips

8.3.3 – Key Points for the Exam

  • Red-shift An increase in observed wavelength of light from a source moving away from the observer.
  • Blueshift A decrease in observed wavelength of light from a source moving towards the observer.
  • Spectral line A narrow band of wavelengths in a spectrum, characteristic of a particular element.
  • Recession Movement of a galaxy away from Earth.
  • Cosmic microwave background Uniform microwave radiation from all directions, left over from the early Universe.

Common Mistakes

  • Saying that red-shift means galaxies look redder; it means the wavelength of the light is stretched.
  • Using the observed wavelength as the reference wavelength in the Doppler equation.
  • Stating that the Universe has a centre because galaxies are receding; expansion has no central point.
  • Saying that the most distant galaxies move more slowly; more distant galaxies recede faster.

Last updated:

Cambridge 9702 & Edexcel Physics Mark Scheme Points

Doppler Shift & Galactic Red-shift Criteria

Key Equation: Δλ / λ ≈ Δf / f ≈ v / c

  • Red-shift: Fractional increase in observed wavelength from receding galaxies.
  • Hubble's Law: v = H0d (Universe age ≈ 1 / H0).
  • Evidence for Big Bang: Cosmological red-shift + CMBR at 2.7K.
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