Sounds travel as longitudinal waves and are produced when something vibrates,
Sounds can't travel through a vacuum but they reflects off hard surfaces to produce echoes. Sound is refracted when it pases into a different medium or subtance. Sound can be diffracted around buildings or land masses.
Frequency and Pitch
The Frequency of a sound wave is the number of vibrations produced in one second. It is measured in hertz (Hz).
Humans can hear sounds in the range 20-20000 Hz. The frequency affects the pitch of the sound.
As the frequency is increased the sound becomes higher pitched.
Amplitude and Loudness
Amplitude is the peak of the movement of the sound from it's rest point. The amplitude affects the loudness of the sound.
As the amplitude is increased the sound becomes louder.
The Doppler Effect
When a wave source is moving towards or away from you there is a change in the wavelength and frequency.
We can detect this change with moving vehicles. For example, a car approaching you quickly produces sound waves that are bunched up and they become higher pitched as the frequency becomes higher. When it moves away the sound waves are stretched out, become lower frequency and lower pitched.
When a light source is moving away or towards us its frequency appears to change.
When we look at light from the sun some freqencies of light are not seen because they are absorbed by hydrogen and helium in the sun. This appears as black lines in the sun's spectrum.
When we look at a galaxy moving away from us the black lines have been shifted to lower frequencies. This is called the red-shift. If the galaxy was moving towards us it would be the blue-shift.
Almost all glaxies show red-shift. The futher away a glaxy is the more red-shifted it is. This means that the more distant the galaxy the faster it is moving away from us.
The Big Bang Theory
The red shift is evidence that the universe might be expanding. Scientists think that if the Universe is expanding then it could have started at a single point and expanded outwards like an explosion. This is the Big Bang Theory.
The Big Bang Theory states that the Universe started from a very small inital point billions of years ago and has been expanding ever since. The point from which i started contained all the space, energy and matter in the Universe.