Starter Physics - Waves (Sound and Light)

Waves (Sound and Light)

Sound and light are examples of waves, where energy is transmitted by oscillations.

We can see waves in everyday life in the sea or the ripples in pools of water.
The surface of the water is not perfectly flat, with some areas higher than others.
We can make a constant stream of waves in water by repeatedly moving our hand or fingers up and down in the surface.
The distance between the waves depends on how frequently we oscillate the hand.
Sound is a type of wave - but instead of water, it moves the air.
Light is another type of wave, but with a moving electromagnetic field.
Waves have properties such as:

Sound moves through the air as a pressure wave, like when you stretch out a slinky then push and pull it rapidly - you see the 'pressure wave' move along the slinky.
A loudspeaker uses a speaker cone to push/pull the air, which is moved by an electromagnet, driven by an electrical signal.
Sound can be reflected off hard surfaces, such as walls.
This is how we can hear an echo if we shout in a big space with large walls, like a cave or a hall.
This tells us that sound waves travel through air at a certain speed (343 metres/sec) which we could measure by the time taken for the echo to reach us.
We hear sounds by detecting the movement of our ear drum when the waves hit it.
Sound is usually transmitted through air, but can also travel through other materials, such as water or solids. The speed is much faster through water or solids.

Light is a wave made up of electromagnetism.
It doesn't need a medium (like air or water) to move in - because electric and magnetic fields work even in a vacuum.
Light is just one type of electromagnetic wave.
Electromagnetic waves include radio waves, microwaves, infra-red light, visible light, UV light, X-rays, gamma rays, etc.
Light and the other electromagnetic waves travels incredibly fast, at 300 million metres/sec, much faster than sound.
This is why we see lightning several seconds before we hear thunder.
Light can reflect off surfaces, especially if they are flat and shiny.
If the surface is rough then the reflected light is more scattered.
Light can pass through certain materials, such as glass.
If the glass is shaped then the light is bent (by what we call refraction).
Lenses are shaped to bend light to a focus, like we see in a magnifying glass, or a telescope.
We can split the light using a prism. This bends the light by different amounts depending on the colour of the light.
This is because the bending of the light depends on its wavelength or frequency, which is it's colour. This is called dispersion.
We detect light with our eyes, which focuses the light using a lens, onto the retina which is covered with red, blue, or green detectors.
Cameras work in the same way, but using photo-sensitive film or electronic detectors.

The electromagnetic spectrum covers a huge range of wavelengths. (Image credit: NASA).