Edexcel iGCSE Physics -8.17P Red-Shift and Distance- Study Notes- New Syllabus
Edexcel iGCSE Physics -8.17P Red-Shift and Distance- Study Notes- New syllabus
Edexcel iGCSE Physics -8.17P Red-Shift and Distance- Study Notes -Edexcel iGCSE Physics – per latest Syllabus.
Key Concepts:
update
Red-Shift of Light from Galaxies at Different Distances
Light received from distant galaxies shows a red-shift. The amount of red-shift depends on how far away the galaxy is from the Earth.
Key Statement
Statement: Light from galaxies that are farther away from the Earth shows a greater red-shift than light from nearer galaxies.
Key idea: Greater distance means greater speed of recession.
What Is Red-Shift?
- Red-shift is an increase in the wavelength of light.
- Spectral lines move toward the red end of the spectrum.
- It occurs when a light source moves away from the observer.
Important: Red-shift is caused by the Doppler effect for light.
Red-Shift and Distance
- Nearby galaxies show small red-shifts.
- Distant galaxies show larger red-shifts.
- Very distant galaxies show extremely large red-shifts.
Conclusion: The further away a galaxy is, the faster it is moving away.
Why Red-Shift Increases with Distance
- The universe is expanding.
- Space itself is stretching.
- More distant galaxies are carried away faster by this expansion.
Key idea: This relationship is evidence that the universe is expanding uniformly.
Mathematical Link (Reminder)
\( \mathrm{\dfrac{\Delta \lambda}{\lambda_0} = \dfrac{v}{c}} \)
- Larger \( \mathrm{\Delta \lambda} \) → larger velocity \( \mathrm{v} \)
- Larger velocity → galaxy is farther away
Observational Evidence
- All distant galaxies show red-shift.
- No distant galaxies show blue-shift.
- This shows the universe is expanding in all directions.
Comparison of Galaxies at Different Distances
| Galaxy distance | Observed red-shift | Speed relative to Earth |
|---|---|---|
| Near | Small | Slow recession |
| Far | Large | Fast recession |
Why This Supports the Big Bang Theory
- Expansion means galaxies were once closer together.
- Tracing expansion backwards leads to a single origin.
- This origin is explained by the Big Bang theory.
Example
Two galaxies are observed. Galaxy A is much farther from Earth than Galaxy B. Describe and explain the difference in the red-shift of light received from the two galaxies.
▶️ Answer / Explanation
- Galaxy A is farther away.
- Galaxy A shows a larger red-shift.
- This means Galaxy A is moving away faster.
- More distant galaxies have higher recessional speeds.
Example
Explain why almost all galaxies observed from Earth show red-shift rather than blue-shift.
▶️ Answer / Explanation
- The universe is expanding.
- Most galaxies are moving away from Earth.
- Motion away increases wavelength.
- This produces red-shift.
- Therefore red-shift is commonly observed.