Sound and light are both waves that carry energy without carrying matter, but they differ in many other ways, including whether they need a medium to travel through.
Water waves are transverse: the water moves up and down while the wave travels forwards. A floating cork bobs on the spot rather than being carried along, which shows that the wave transfers energy, not water.
Waves reflect off a barrier, and when two waves meet they superpose: where two crests coincide the displacement adds and the wave is bigger, and where a crest meets a trough they cancel.
Sound is produced by something vibrating. The vibration pushes the air into compressions and rarefactions, so sound is a longitudinal wave: the particles move back and forth along the direction the wave travels.
It is detected when those pressure changes make something else vibrate, such as an ear drum or the diaphragm of a microphone.
Frequency is the number of vibrations each second, measured in hertz (Hz), and it determines the pitch of a sound. A higher frequency sounds higher.
Sound reflects off hard surfaces, and a reflection heard back at the source is an echo. Soft, porous materials absorb more sound and reflect less, so a carpeted room sounds quieter than a bare one.
Sound cannot travel through a vacuum, because there are no particles to pass the vibration on. A ringing bell in a jar goes silent as the air is pumped out.
It travels faster in liquids than in gases, and faster still in solids, because the particles there are more strongly coupled and pass the disturbance on sooner: about 330 m/s in air and about 1500 m/s in water.
Humans can hear roughly 20 Hz to 20 000 Hz, and the upper limit falls with age. Sound above that range is ultrasound.
Many animals hear beyond our range: dogs and bats detect much higher frequencies, and elephants communicate below it.
Because sound is a pressure wave it carries energy, and that energy can be put to work. Ultrasound is used to clean delicate objects and in physiotherapy.
It also carries information. A microphone converts the pressure variations into an electrical signal that matches them, which is how sound is recorded and transmitted.
Both are waves and both transfer energy, and both can be reflected and absorbed. But light is transverse and sound is longitudinal, light travels enormously faster, and, most importantly, light does not need a medium.
That is why you see lightning before you hear thunder, and why the Sun lights us across empty space while the explosion of a distant star is silent.
Light travels through a vacuum at about \(3\times10^{8}\) m/s, or 300 000 000 metres every second, the fastest anything can travel.
Even at that speed, distances in space are so vast that light from the Sun takes about eight minutes to reach us.
When light meets a material it may be transmitted, absorbed or reflected, and usually some of each.
A smooth surface gives specular reflection, producing a clear image, as in a mirror. A rough surface gives diffuse scattering, sending light in all directions. A sheet of paper can therefore be seen from any angle, while a mirror gives a reflection from one direction only.
Light is drawn as straight rays with arrows. At a plane mirror the angle of incidence equals the angle of reflection, both measured from the normal, and the image appears as far behind the mirror as the object is in front.
Refraction is the bending of light when it changes speed entering a new material. It is why a straw looks broken at the water surface. A convex lens brings rays together at a focus, which is how the lens in the eye and in a camera forms an image on the surface behind it.
Light carries energy from a source to whatever absorbs it. Absorbed light can produce a chemical effect, as in photosynthesis or photographic film, or an electrical effect, as in a solar cell.
The light-sensitive cells of the retina work the same way, turning absorbed light into signals the brain can read.
White light is a mixture of colours of different frequencies. A prism refracts each colour by a different amount and spreads them into a spectrum: red, orange, yellow, green, blue, indigo, violet.
In white light, an opaque object looks the colour of the light it reflects most strongly, absorbing the other wavelengths. A red jumper reflects red most strongly; a white one reflects all wavelengths roughly equally; a black one absorbs most of the light falling on it.
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