IB DP Physics: HL复习笔记9.5.2 Uses of The Doppler Effect
Uses of The Doppler Effect
- The?Doppler effect?is important in many key areas of science including:
- Radar?readings for moving objects
- Measuring the?rate of blood flow?within a patient
- Finding planet?orbits?around distant stars
- Mapping the?expansion?of the universe
Redshift of EM Radiation
- On Earth, the Doppler effect of sound can be easily observed when sound waves move past an observer at a notable speed
- In space, the Doppler effect of light can be observed when spectra of distant stars and galaxies are observed, this is known as:
- Redshift?if the object is moving away from the Earth, or
- Blueshift?if the object is moving towards the Earth
- Redshift is defined as:
The fractional increase in wavelength (or decrease in frequency) due to the source and observer receding from each other
- For?non-relativistic?galaxies, Doppler redshift can be calculated using:
- Where:
- Δλ = shift in wavelength (m)
- λ = wavelength emitted from the source (m)
- Δf = shift in frequency (Hz)
- f?= frequency emitted from the source (Hz)
- v?= speed of recession (m s-1)
- c?=?speed of light?in?a vacuum (m s-1)
Worked Example
The spectra below show dark absorption lines against a continuous visible spectrum.
A particle line in the spectrum of light from a source in the laboratory has a frequency of 4.570 × 1014?Hz. The same line in the spectrum of light from a distant galaxy has a frequency of 4.547 × 1014?Hz.
Calculate the speed of the distant galaxy in relation to the Earth. Determine whether it is moving towards or away from the Earth.
Step 1:?Write down the known quantities
-
- Received frequency,?fr?= 4.547 × 1014?Hz
- Original frequency,?f0?=?4.570 × 1014?Hz
- Shift in frequency, Δf?= (4.547 – 4.570) × 1014?= –2.3 × 1012?Hz
- Speed of light,?c?=?3.0 × 108?m s–1
Step 2:?Write down the Doppler redshift equation
Step 4:?Write a concluding sentence
The observed frequency is?less?than the emitted frequency (the light from a laboratory source), therefore, the source is?receding, or moving away, from the Earth at?1.5 × 106?m s–1
An Expanding Universe
- After the discovery of Doppler redshift, astronomers began to realize that almost all the galaxies in the universe are receding
- This led to the idea that the space between the Earth and the galaxies must be?expanding
- This expansion stretches out the light waves as they travel through space, shifting them towards the red end of the spectrum
- The more red-shifted the light from a galaxy is, the?faster?the galaxy is moving away from Earth
- The expansion of the universe can be compared to dots on an inflating balloon
- As the balloon is inflated, the dots all move away from each other
- In the same way as the rubber stretches when the balloon is inflated, space itself is?stretching out between galaxies
- Just like the dots, the galaxies move away from each other, however, they themselves do not move
转载自savemyexams
国际竞赛真题资源免费领取
美高学分项目重磅来袭！立即了解
