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International Baccalaureate · IB Diploma · Physics · HL หัวข้อ 15 8:06 การบรรยายภาษาอังกฤษ · คำบรรยายภาษาอังกฤษ + 中文 ลอยตัวบนภาพ
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Transcript
A recognizable spectral line appears at a longer wavelength in a distant source.
遥远天体中可识别的光谱线出现在更长的波长处。
Its shift can provide evidence of recession along the line of sight.
这种位移可为沿视线方向的退行提供证据。
Relative source-observer motion changes observed frequency or wavelength.
相对运动源与观测者之间的相对运动会改变观测到的频率或波长。
For light at a relative speed much smaller than c, the magnitude of the fractional wavelength shift is approximately v over c.
对于相对于光速c速度远小于c的光，其分数波长的变化量级近似为v除以c。
Recession corresponds to longer wavelength, and approach to shorter wavelength.
退行对应更长的波长，接近对应更短的波长。
Redshift: Shift of an identified line toward longer wavelength.
红移：已识别谱线向更长波长方向的移动。
Rest wavelength: Wavelength of the line measured with no relative source motion.
静止波长：在源与观测者无相对运动时测得的谱线波长。
Compare an identified spectral line with its laboratory rest wavelength.
将识别出的谱线与其实验室静止波长进行比较。
Use z equals observed minus rest, all over rest.
使用公式 z 等于观测值减去静止值，再除以静止值。
Then v is approximately c times z in the stated low-speed model.
然后在所述低速模型中，v 约等于 c 乘以 z。
Do not apply this approximation without checking the regime or treating a cosmological redshift as a simple exact velocity.
在检查适用条件之前，不要应用此近似，也不要将宇宙学红移简单视为精确速度。
In the lab: Use attributed spectra with calibration and line identification.
在实验室中，使用经标定和谱线识别的归属光谱。
Compare several lines for a consistent shift.
比较多条谱线以确认一致的移动。
State uncertainty and distinguish relative line-of-sight motion from an unmeasured transverse component.
说明不确定度，并区分相对视线运动与未测量的横向分量。
Known: a 500 nanometre rest line is observed at 501 nanometre.
已知：一条500 nm的静止谱线被观测为501 nm。
Z equals 501 minus 500, all over 500, which equals 0.002.
z等于501减500，除以500，即0.002。
With C equal to 3.0 times 10 to the 8th power metres per second, V is approximately 6.0 times 10 to the 5th power metres per second away.
取c等于3.0乘10的8次方米每秒，v约为6.0乘10的5次方米每秒，方向远离。
The sound source denominator formula is not the required special relativity light approximation.
声源分母公式并非本题所需的狭义相对论光近似式。
Your turn.
轮到你了。
A 600 nanometre rest line is observed at 601.2 nanometres.
一条静止波长为 600 纳米的谱线在 601.2 纳米处被观测到。
Find z, the observed wavelength minus the rest wavelength, divided by the rest wavelength.
求 z，即观测波长减去静止波长，再除以静止波长。
The approximate shift relation does not show that light travels faster from an approaching source.
近似位移关系并不能表明来自接近光源的光传播得更快。
A single unidentified line cannot safely establish a redshift.
仅凭一条未识别的谱线，无法安全地确认红移。
Not true: An observed redshift means light speed increased.
不正确。 观测到的红移并不意味着光速增加。
Light speed in vacuum is the same for all observers.
真空中光速对所有观测者而言是相同的。
Redshift is a change in observed wavelength from relative motion (or expanding space), not light travelling faster or slower.
红移是由于相对运动或空间膨胀导致的观测波长变化，而非光传播速度变快或变慢。
Relative source-observer motion changes observed frequency or wavelength.
波源与观察者的相对运动改变了观测到的频率或波长。
Compare a identified line with its laboratory rest wavelength.
将识别出的谱线与实验室静止波长进行比较。
Watch for: the approximate shift relation does not show that light travels faster from an approaching source.
注意：近似的频移关系并不能说明来自接近光源的光传播得更快。
A four hundred nanometre rest line is observed at four hundred and one point six nanometres.
一条400 nm的静止谱线被观测为401.6 nm。
Find z.
求z。
Redshift does not mean the speed of light increased.
红移并不意味着光速增加了。
The result is a redshift of 0.004.
结果是红移为0.004。
Find a known line, measure how far it has moved, and divide by its rest wavelength to get z.
找到一条已知谱线，测出它移动的距离，再除以静止波长得到z。
At low speed the recession speed is about c times z.
低速时，退行速度约等于光速乘以z。
A 500 nanometre rest line is observed at 499 nanometres.
一条静止波长为 500 纳米的谱线，观测到的波长为 499 纳米。
Find z and the low-speed radial velocity, using c equal to 3 times 10 to the 8 metres per second. Explain the sign and the limit.
取 c 等于 3×10⁸ 米每秒，求 z 和低速近似下的径向速度，并解释符号和适用范围。
z is minus 0.002, so v is about c times z, which is minus 6 times 10 to the 5 metres per second: approaching, under the positive-away convention.
z 等于 −0.002，所以 v 约等于 c 乘以 z，即 −6×10⁵ 米每秒：按“远离为正”的约定，表示正在接近。
This small-z kinematic approximation does not cover every origin of redshift.
这种小 z 的运动学近似并不适用于所有红移来源。
An approaching sound source is heard at a higher pitch even when the source produces a constant frequency.
即使声源产生恒定频率，靠近的声源听起来音调也会更高。
Doppler shift arises from relative source-observer motion.
多普勒频移源于波源与观察者之间的相对运动。
A moving source compresses wavefront spacing ahead and increases it behind.
运动的波源会使前方的波面间距压缩，后方的波面间距增大。
For sound in a stationary medium, the wave speed relative to that medium need not change.
对于静止介质中的声波，相对于该介质的波速不必改变。
Doppler shift is the change of observed frequency due to relative motion.
多普勒频移是指由于相对运动引起的观测频率变化。
Wavefront is a surface joining points with the same wave phase.
波面是连接具有相同波相位的点所构成的曲面。
For a source approaching a stationary observer along the line of sight, f observed equals f source times v divided by v minus v source.
当波源沿视线方向接近静止观察者时，观测频率等于波源频率乘以v除以v减v源。
Recession uses v plus v source in the denominator.
退行情况分母使用v加v源。
These are sound wave models; light requires its appropriate relation.
这些是声波模型；光需要其相应的关系式。
In the lab: Use a simulation or recorded data with stated source and observer directions.
在实验室中，使用具有明确声明的波源和观察者方向的模拟或记录数据。
Draw wavefronts before choosing signs.
在选择符号之前绘制波前。
Avoid demonstrations near moving traffic or unsafe high-volume sources.
避免在移动车辆附近或不安全的高音量声源旁进行演示。
Known: source frequency 500 Hz, sound speed 340 metres per second, and the source approaches at 20 metres per second.
已知：声源频率 500 Hz，声速 340 米每秒，声源以 20 米每秒的速度靠近。
The observed frequency is 500 times 340, divided by 340 minus 20, which is 531.25 Hz.
观测频率等于 500 乘以 340，再除以（340 减 20），即 531.25 Hz。
The increase is 31.25 Hz; it is not the source frequency changing.
增加了 31.25 Hz；这不是声源频率本身发生了变化。
Your turn.
轮到你了。
A 600 hertz source approaches a stationary observer at 20 metres per second.
一个频率为 600 赫兹的波源以每秒 20 米的速度接近静止的观察者。
Use v equals 340 metres per second.
取声速 v 等于每秒 340 米。
Find the observed frequency.
求观察到的频率。
A stationary source and moving observer require the observer-motion relation.
静止波源与运动观测者需使用观测者运动关系式。
Do not use the same denominator formula for every arrangement or import a sound formula into relativity.
切勿对每种情形都套用同一分母公式，也不可将声速公式直接引入相对论中。
Not true: The Doppler effect always means the wave speed in the medium changed.
这种说法不正确。 多普勒效应并不总是意味着介质中的波速发生了变化。
The wave speed in the medium is fixed by the medium.
介质中的波速由介质本身决定，是固定的。
The Doppler shift comes from relative motion changing the spacing of wavefronts (moving source) or the rate they meet the observer (moving observer) - not from a change in wave speed.
多普勒频移源于相对运动改变了波前的间距（对于运动的波源），或改变了它们与观测者相遇的速率（对于运动的观测者），而非波速改变所致。
Doppler shift arises from relative source-observer motion.
多普勒频移源于波源与观察者之间的相对运动。
For a source approaching a stationary observer along the line of sight, f observed equals f source times v divided by v minus v source.
当波源沿视线方向接近静止的观察者时，f观察等于f源乘以v除以v减v源。
Watch for: A stationary source and moving observer require the observer-motion relation.
注意：静止波源和运动观察者需要应用观察者运动的公式。
A 450 Hz source approaches a stationary observer at 40 m/s.
一个频率为 450 Hz 的波源以 40 m/s 的速度接近静止的观察者。
The wave speed is 340 m/s.
波速为 340 m/s。
Find the observed frequency.
求观测到的频率。
The wave speed in the medium does not change.
介质中的波速保持不变。
The result is an observed frequency of 510 hertz.
结果是观测频率为510赫兹。
As the source approaches, its waves are squeezed together and the observed frequency rises.
声源靠近时，波被压缩，观测到的频率升高。
With sound at 340 metres per second, a 500 hertz source at 10 metres per second is heard at about 515 hertz.
声速为每秒340米时，以每秒10米靠近的500赫兹声源听起来约为515赫兹。
A 600 Hz sound source recedes at 20 m/s from a stationary observer; sound speed is 340 m/s.
一个频率为 600 Hz 的声源以 20 m/s 的速度远离静止观察者。 声速为 340 m/s。
Find frequency and compare approach.
求接收到的频率，并与接近情况作比较。
Why is this not the light Doppler law?
为什么这不是光的多普勒定律？
Recession gives 600 times 340, divided by 340 plus 20, which is 566.7 Hz; approach gives 637.5 Hz.
远离时，频率等于 600 乘以 340，再除以（340 加 20），即 566.7 Hz；靠近时为 637.5 Hz。
Source direction is relative to a material medium; light requires its appropriate relativistic relation.
声源的运动方向是相对于介质而言的；光需要使用相应的相对论关系。
Pause and check yourself. Explain each point on screen in your own words, then change one number in the last example and say which assumption must still hold.
暂停一下，自我检查：用自己的话解释屏幕上的每个要点，再改动最后一个例子中的一个数字，说明哪个假设仍然必须成立。