Waves: one set of rules for sound, light and everything else
A wave carries energy without carrying the stuff it travels through.
A wave is a disturbance that transfers energy without transferring matter. Drop a stone in a pond and the ripple travels outward, but the water does not - a floating leaf bobs up and down and stays where it was.
That distinction is the whole idea, and it is why one set of rules covers sound, light, radio, earthquakes and the sea.
The vocabulary
- Wavelength - distance between one peak and the next, in metres
- Frequency - peaks passing a point each second, in hertz
- Amplitude - height of the peak, which determines loudness or brightness
- Speed - how fast the wave travels, which is set by the medium
They are tied together by one equation: speed = frequency x wavelength. Since the speed is fixed by what the wave is travelling through, frequency and wavelength trade off against each other. Higher frequency always means shorter wavelength.
Two kinds
- Transverse - the disturbance is at right angles to the travel. Light, and a rope shaken up and down.
- Longitudinal - the disturbance is along the direction of travel, as compressions and rarefactions. Sound.
Because sound is a squeeze-and-stretch of a material, it needs a material. In a vacuum there is nothing to compress, so there is no sound. Light needs nothing, which is why we can see the Sun but not hear it.
Visible light is a narrow strip of one continuous spectrum - our eyes evolved to use the band the Sun emits most of, and that the atmosphere lets through. There is nothing physically special about it.
What waves do
Reflection
Bounce off a surface, angle in equal to angle out. Mirrors, echoes, radar, ultrasound scans.
Refraction
Change speed when entering a new medium, and bend if they arrive at an angle. This is why a straw looks broken in a glass, how lenses work, and why a pool is deeper than it appears. Different wavelengths bend by slightly different amounts, which is what splits white light into a rainbow.
Diffraction
Spread out when passing an edge or a gap, most noticeably when the gap is about the size of the wavelength. Sound diffracts around door frames easily because its wavelength is around a metre; light barely does, because its wavelength is under a thousandth of a millimetre. That is why you can hear round a corner but not see round one.
Interference
Two waves overlapping add where peaks meet peaks and cancel where a peak meets a trough. Noise-cancelling headphones generate the exact opposite of the incoming sound so the two cancel.
The Doppler effect. A source moving toward you compresses the waves ahead of it, raising the frequency; moving away, it stretches them. An ambulance drops in pitch as it passes. The same effect in light is how we know distant galaxies are moving away - their light is shifted toward the red, longer-wavelength end.
Why the sky is blue
Air molecules scatter short wavelengths far more strongly than long ones - the effect goes roughly as one over wavelength to the fourth power, so blue scatters about ten times more than red.
Look anywhere but at the Sun and you see scattered light, which is mostly blue. At sunset the light travels through far more atmosphere, so most of the blue has been scattered away before it reaches you and what is left is red and orange. Same physics, different path length.