Waves
OxfordAQA · International GCSE · Physics Topic 3 6:01 English narration · English + 中文 subtitles burned in
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Waves move energy from a source to a receiver without moving matter: light to an eye, sound to an ear, radio to a phone.
波把能量从波源传到接收者，而不移动物质：光传到眼睛，声音传到耳朵，无线电波传到手机。
This topic builds the wave model, frequency, wavelength and speed, then uses it for the electromagnetic spectrum, sound, reflection and refraction.
本主题建立波的模型：频率、波长和波速，然后用它来解释电磁波谱、声音、反射和折射。
Transverse waves oscillate across the direction of travel, like light and water waves.
横波的振动方向与传播方向垂直，例如光和水波。
Longitudinal waves oscillate along it, making compressions, where the particles crowd together, and rarefactions, where they spread out.
纵波的振动方向与传播方向平行，形成疏部和密部：密部是粒子挤在一起的地方，疏部是粒子分散开的地方。
Sound is longitudinal.
声波是纵波。
Diffraction is the spreading of waves through a gap.
衍射是波通过缝隙时的展开。
Through a wide gap the wavefronts pass almost straight, with little spreading.
缝隙很宽时，波阵面几乎直线通过，展开很少。
When the gap is close to the wavelength, they spread out strongly, in curved wavefronts.
缝隙宽度接近波长时，波以弯曲的波阵面强烈地展开。
Wave speed equals frequency times wavelength.
波速等于频率乘以波长。
A water wave of 2 hertz and wavelength 1.5 metres travels at 3 metres per second.
频率 2 赫兹、波长 1.5 米的水波，速度是每秒 3 米。
A radio station at 100 megahertz: every electromagnetic wave travels at 3.0 times 10 to the 8 metres per second, so the wavelength is 3.0 metres.
100 兆赫的电台：所有电磁波的速度都是每秒 3.0 乘 10 的 8 次方米，所以波长是 3.0 米。
The graph shows three complete waves in 1.20 metres, so the wavelength is 0.40 metres: check it crest to crest.
图中 1.20 米内有三个完整的波，所以波长是 0.40 米：用相邻波峰检查。
The amplitude is from the rest position up to a crest, 3.0 centimetres, not the 6.0 from crest to trough.
振幅是从平衡位置到波峰，3.0 厘米，而不是波峰到波谷的 6.0 厘米。
Five waves every 2.0 seconds is 2.5 hertz, so v is 1.0 metre per second.
每 2.0 秒通过五个波，频率是 2.5 赫兹，所以波速是每秒 1.0 米。
The electromagnetic spectrum is one family: all transverse, all travelling at the same speed in a vacuum.
电磁波谱是一个家族：都是横波，在真空中的速度都相同。
From radio waves, longer than 10 to the 4 metres, through microwaves, infrared, visible light and ultraviolet, to X-rays and gamma: the frequency rises as the wavelength falls.
从波长超过 10 的 4 次方米的无线电波，经过微波、红外线、可见光和紫外线，到 X 射线和伽马射线：频率升高，波长变短。
Each band has uses and hazards.
每个波段都有用途和危害。
Radio carries TV, radio and Bluetooth.
无线电波传送电视、广播和蓝牙信号。
Microwaves serve phones and satellite TV, but heat tissue.
微波用于手机和卫星电视，但会加热人体组织。
Infrared drives remotes and night vision, and can burn.
红外线用于遥控器和夜视仪，也会造成灼伤。
Ultraviolet marks property, and causes skin cancer.
紫外线用于防伪标记，会导致皮肤癌。
X-rays and gamma image and sterilise, but kill cells.
X 射线和伽马射线用于成像和消毒，但会杀死细胞。
Every object emits and absorbs infrared.
所有物体都发射和吸收红外线。
Hotter objects radiate more, and at higher frequencies: black-body radiation.
物体越热，辐射越多，频率越高：这就是黑体辐射。
Dark, matt surfaces are the best absorbers and the best emitters; light, shiny surfaces reflect infrared instead.
深色、粗糙（无光泽）的表面是最好的吸收体和发射体；浅色、光亮的表面则反射红外线。
X-rays have a very short wavelength and high energy, and they ionise.
X 射线波长很短、能量很高，具有电离性。
Metal and bone absorb them, soft tissue transmits them, and they darken photographic film, so a fracture shows as a dark line through white bone.
金属和骨骼吸收它，软组织让它透过，它还能使照相底片变黑，所以骨折在白色的骨头上显示为一条暗线。
The same rays are used in CT scans and to treat cancer.
同样的射线也用于 CT 扫描和癌症治疗。
Sound is a longitudinal wave that vibrates a medium.
声音是使介质振动的纵波。
Humans hear between 20 hertz and 20 000 hertz.
人能听到的频率在 20 赫兹到 20 000 赫兹之间。
Pitch follows frequency: higher frequency, higher note.
音调由频率决定：频率越高，音越高。
Loudness follows amplitude.
响度由振幅决定。
Ultrasound is sound above 20 kilohertz, too high for us to hear.
超声波是高于 20 千赫的声音，人耳听不到。
A ship sends an ultrasound pulse to the sea bed and hears the echo 0.40 seconds later.
一艘船向海底发出超声波脉冲，0.40 秒后听到回声。
In sea water sound travels at 1500 metres per second, so the pulse covers 600 metres there and back.
声音在海水中的速度是每秒 1500 米，所以脉冲往返共走了 600 米。
Divided by two, the sea is 300 metres deep.
除以二，海深 300 米。
Draw the normal, a line at right angles to the mirror where the ray strikes.
在光线射到镜面的位置画法线，即与镜面垂直的线。
The angle of incidence, between the incoming ray and the normal, equals the angle of reflection.
入射光线与法线的夹角是入射角，它等于反射角。
A plane mirror gives a virtual, upright image, laterally inverted.
平面镜成的像是虚像、正立、左右颠倒。
Light enters a glass block with angle of incidence 40 degrees.
光以 40 度的入射角进入玻璃砖。
It slows down and bends towards the normal, to 25 degrees.
它减速并向法线偏折，折射角为 25 度。
The refractive index is sine 40 over sine 25, which is 1.52, so light in this glass travels at 3.0 times 10 to the 8 divided by 1.52, 2.0 times 10 to the 8 metres per second.
折射率等于 sin 40 除以 sin 25，即 1.52，所以光在这种玻璃中的速度是 3.0 乘 10 的 8 次方除以 1.52，即每秒 2.0 乘 10 的 8 次方米。
Inside the glass, a ray meets the surface at 50 degrees to the normal.
在玻璃内部，一条光线以与法线成 50 度的角射到表面。
The critical angle for this glass is 41 degrees, from sine c equals one over n.
由 sin c 等于 1 除以 n，这种玻璃的临界角是 41 度。
Fifty is more than 41, so no light gets out: the ray is totally internally reflected.
50 度大于 41 度，所以光出不去：发生全反射。
Optical fibres trap light this way.
光纤正是这样把光困在里面。
A convex lens brings parallel rays to the principal focus.
凸透镜把平行光会聚到主焦点。
To find an image, draw two rays from the top of the object: one parallel to the axis, which then passes through the focus, and one through the centre, undeviated.
求像时，从物体顶端画两条光线：一条平行于主光轴，折射后经过焦点；一条经过光心，方向不变。
Where they meet is the image: here real and inverted.
两条光线的交点就是像：这里是倒立的实像。
The cornea does most of the refracting; ciliary muscles and suspensory ligaments change the lens's shape to focus on the retina.
角膜承担大部分折射；睫状肌和悬韧带改变晶状体的形状，把像聚焦在视网膜上。
In long sight the eyeball is too short, corrected with a convex lens.
远视眼的眼球太短，用凸透镜矫正。
In short sight it is too long, corrected with a concave lens.
近视眼的眼球太长，用凹透镜矫正。
Four common mistakes. Not halving the distance in an echo question. Measuring angles from the surface instead of the normal. Saying total internal reflection happens as light enters glass: it happens inside the denser medium. And correcting short sight with a convex lens.
四个常见错误：回声题中没有把距离除以二；从表面而不是从法线量角度；说全反射发生在光进入玻璃时：它发生在光密介质内部；以及用凸透镜矫正近视。
Check yourself.
自我检测。
A 500 hertz wave of wavelength 0.68 metres travels at 340 metres per second.
频率 500 赫兹、波长 0.68 米的波，速度为每秒 340 米。
Diffraction is strongest when the gap is about one wavelength.
缝宽约等于一个波长时衍射最明显。
Humans hear from 20 hertz to 20 kilohertz.
人能听到 20 赫兹到 20 千赫的声音。
For glass with n of 1.5, the critical angle is about 41.8 degrees.
n 为 1.5 的玻璃，临界角约为 41.8 度。